I Have Airborne Feedback And Never Realized It...Till Now

  While my ZYX Airy is out for a rebuild, I hooked up my CAL cd player/transport and started playing CD`s that I had recorded from vinyl using a Tascam 900.
When I do the recording, nothing is on but the TT setup and the recorder. Room is dead silent. No speakers

I`ve  been listening to a disc or two over the last few nights.

Last night, I was listening to a CD I made of Lindsey Stirling`s 'Shatter Me' LP
I was hearing so many odd/different sounds that I never picked up on before using the TT.

For example, I heard growling sounds (seriously) back ground noises and other THINGS that all were hidden when I was playing the TT.
This LP is Bass Heavy! Lots of energy in the air. With 3 15" subs I know that.

My TT is pretty much isolated IMO
I use a Rega wall mount bracket that is bolted to my equipment rack not the wall.
I have the TT sitting on a SRM isolation platform that sits on the Rega bracket
Concrete slab floor.

No doubt the cartridge is picking up on all energy that and resubmitting it.

This won`t be an easy fix I`m afraid..  :(

You are on the right track but I seriously doubt it is acoustic, it is almost certainly mechanical. Speakers on spikes transmit energy right through the floor, up the wall and into your turntable. Same happens with CD just nowhere near as obvious. When it is gone you will hear it though even with CD. 

Concrete floor, one of the great misconceptions is you cannot have these problems because, concrete floor. But concrete transmits vibrations just fine, only the frequency and speed of transmission varies. There is nothing inherently vibration damping or absorbing about concrete. Quite the opposite. Rings like a bell. 

So your solution is very simple, what you need is to isolate the speakers from the floor, and the turntable from the rack. The SRM you are using is kinda sort okay, better than nothing, but obviously not enough.  

There's a couple ways you can go. Least expensive will be something like Nobsound springs under your turntable or under the SRM. Not sure which one you have. The one I found on-line is not very impressive but it could work as a shelf with springs under it. You can use these springs under the speakers as well. Very effective. Surprisingly effective for the money.  

If you can afford it however the best solution will be Townshend Pods and Podiums. Which is best will depend on the size and weight of your Rega. Podiums for sure will be best under your speakers. The vast majority of your problem is coming from direct transmission of vibrations from the speakers through the floor and walls into the turntable. I had this even with my massive 750lb turntable rack. Dramatically improved clarity and detail. 

Not cheap but will totally solve your problem, while taking your whole system up a step in the process. Or if you want to try Nobsound springs do a search, there was a very informative thread on them last year. I used them before going to Townshend, they really do work, quite well in fact, just nowhere near as good as Pods and Podiums. 
I remember back when I used to record my lp`s to cassette, I could very   loudly talk right at the cartridge while recording and I could hear my voice  on the tape afterwards.
That's what is happening here I`m pretty sure.
It's not mechanical IMO

"Bolted to your equipment rack"....full stop...
Your biggest issue is right there IMO.... its doing nothing there....might as well be sitting directly on the rack or the floor for that matter ... 
The rack is very stable and there`s less vibration on it than being bolted to a large vibrating wall.
In my previous home my components sat on two shelves fitted into a 5' wide alcove along a side wall in the listening room.  I happened to have placed the turntable at the far right end of the upper shelf.

One day I happened to lean over the turntable while a CD was playing with some reasonable bass content.  With my head in that corner of the alcove I was surprised by the bass loading.  So I thought, that can't be good for the cartridge/stylus to be playing in that location.  After I moved the table away from the corner to where the arc traced by the tonearm was near the mid-point of the shelf playback became cleaner and with increased definition.  

That was an acoustic loading problem.
I remember back when I used to record my lp`s to cassette, I could very  loudly talk right at the cartridge while recording and I could hear my voice on the tape afterwards.
That's what is happening here I`m pretty sure.
It's not mechanical IMO

So you're sure the one rules out the other. Interesting.

If you have concrete floors at foundation level you are good to go there.

Turntable just needs a solid foundation on top of that. and that should cover it.

For example my table (Linn Axis...pretty robust on its own) sits on a low solid oak wooden table on top of concrete foundation with thin dense carpet and padding. Rock solid! Nothing picked up from speakers or sub between which it sits. First time ever I’ve achieved that! Never had the solid concrete foundation to work with prior.

I`m sure things would improve if I could put the TT in another room but that would be impractical.

Fortunately I listen at very modest levels..
Maybe, I can put some treatment around the TT and see how that goes.

Dear @scm  : Looks as you are rigth, it's through the air and picked up by the cartridge/tonearm.

As you said it it's not easy to fix it other than change the TT positon or even its shape/angle position.

Regards and enjoy the MUSIC NOT DISTORTIONS,
Here is Max Townshend stamping his foot on the floor, proving once and for all vibrations travel just fine through solid concrete. https://youtu.be/BOPXJDdwtk4?t=86 

I am constantly astounded at the ability of people to ignore solid evidence and persist in wrong thinking, even to the point of talking about moving a turntable to a whole different room, rather than just try what has actually been shown to work. The guy repeating the concrete good to go BS knows it is BS, but keeps repeating it. The guy talking about moving his turntable to another room can solve his problem with some $30 Nobsound springs or even a $5 box of sand. But no, the Good Ship Nonsense has a full head of steam, full speed ahead! Absolutely amazing.
I`m talking about feed back/added content that`s coming through the air and getting picked up, not mechanical feed back like floor bounce, footfalls etc..

If so then you need a room treatment, order acoustic analysis of your room and buy absorbers, diffusers and bass traps for your room.

Concrete floor is the worst material for listening room, if your walls are similar then it’s terrible.
He can stamp on my concrete all he wants and nothing will happen.  Guarandamteed.  
I have speakers in unfinished area with bare concrete floor. That has issues.
But where my good gear is in finished area with thin dense carpet and padding that works best.
Upstairs on suspended plywood floor different issues.
These are the facts. As usual ymmv.  Depends on what the gear is resting on as well. 
I have some room treatments. I have four..4" thick 2'x4' panels and two 2" thick 2'x4' panels that I use at the first reflection points.
I have a carpet that`s 8' x 10' on the tile floor.
I could probably use more treatment.
Good, normally you need much more and not only at first reflection points, probably you need some serious bass traps. 
mapman..." He can stamp on my concrete all he wants and nothing will happen.  Guarandamteed.  "

I`m contemplating adding 2 more subs which, in theory, should eliminate the need for a lot of bass traps scattered about 

I kinda envy the headphone crowd on certain levels and have never thought about that before until now  
The clarity and detail they must hear in a nice setup with really nice headphones must be pretty amazing.
No room resonance, no outside interference at all, none of that.

"The rack is very stable and there`s less vibration on it than being bolted to a large vibrating wall."

The wall racks are meant for an outer weight load bearing wall. If you have a large vibrating wall sitting on concrete it's not an outer wall.....if it is....buy a new house....😜
I'd be willing to bet that vid that miller posted if it was your rack the amplitude running up and out it would shock you coming up through the concrete floor...
If it's airborne your dealing with  ( sounds like it ) you have bigger issues with the room than where the table is....and yes the guys with a nice headphone set up and that's how they listen....It certainly has its positives to it with less negatives to run you around...those high school days late night bedroom listening with headphones at the start of the 70s were much better than just necessity when one thinks about it.....relative to the hoops we jump through now....
The two subs....it can work exceedingly well when done right...
Have to say the Townshend guy’s videos make an awful lot of sense. MC is making a lot of sense on this too. Slap your table on one of those platforms and I bet most problems go away - unless maybe your table’s sitting in a bass node/resonance/peak spot for your room? How much have you experimented with moving your rack’s position (I mean just a bit, not into a different room)?

I’ve been into high-end headphones for a while and they’re wonderful but they can’t match nor eclipse a great speaker/vinyl system IMO. The problem is dealing with room acoustics and vibration feedback. When I had a SOTA Star/Nova table (built-in spring suspension tuned exactly for itself), the vibration feedback into it was literally never any worry. Things got a LOT harder when I got a Clearaudio table. But still, I found I didn’t have any airborne feedback issues of note, even with my HUGE Canterbury speakers played loud, once the ground & structure borne vibration was addressed via rack/etc. Then again I also don’t have 3 subs (I know they are supposed to smooth out bass nodes, not make them worse)!
Have to say the Townshend guy’s videos make an awful lot of sense. MC is making a lot of sense on this too. Slap your table on one of those platforms and I bet most problems go away

Right. Here's some more common sense. 

Ever notice how you can hear the music even without the turntable being hooked up or anything turned on? The stylus tracking the groove causes the whole cartridge body and arm to vibrate so much you can hear it.  

It's not that airborne vibrations aren't making it back into the signal. They are. It's that they are orders of magnitude lower in amplitude than the mechanical vibrations already going on. The sound you hear tracking a record are mechanical in nature. You hear the sound, but the source of the sound is mechanically tracking the groove.   

Still more common sense. Airborne vibrations are super easy to deal with. You can hold a 5 gal plastic bucket over the table, this all by itself will eliminate the majority of the sound reaching the table. Add a little acoustic damping material, drop what's left 90%. People talking about a whole different room simply are not thinking things through. All you need is a good dust cover. But one that is not mechanically connected to the turntable.  

I would take a large plastic tote, big enough to cover the whole turntable, line it with OC703. Cut some strips of OC703, place them on the rack. Set the cover on the strips. This will seal the table off from airborne vibration, while the strips will decouple the tote from the rack and the turntable. This will be the equivalent of an isolation room for the turntable. 

Make it big enough so when you get Pods or a Podium for the turntable you can still use the acoustic cover, and you will have a really sweet isolated rig.
My two öre.
There is calculations regarding tone arm mass, cartridge weight, compliance and so on.
That is for calculating the match between the cartridge and tonearm.
The calculation gives you a low Hz number as output. That is the resonance frequency that the whole assembly.
You want that frequency be out of the hearing range. If you are unlucky maybe you have not as optimal match there.

Second thing is that you can also stand and jump in front of your TT if you turn of the subs. What I trying to say it is not much worth if someone is telling that they can jump beside their TT IF they do not have subwoofers that that play easily down to 10Hz and doing that with some extra dB (not a -3db reading)

With your 3 15" subs you can get into that situation.

So first thing is to check your cartridge and tonearm. If that is a issue try to fix that so you get a frequency in the proper range. (We can’t work against physics).

Then there is unfortunately something called rumble filter that is a hi-pass at somewhere 15 - 20hz (depending on the above).

I implemented that in my miniDSP with a 24 oct/dB slope. (The miniDSP is in my opinion to noisy but therefore it is only used to my two 18" subwoofers there it performs 6 different duties. And this is one of them.)

Yes rumble is a nasty feedback loop noise. So you need to have speakers that play low down with adequate of db to yours tonearm assembly resonance frequency if you do that and a tell is when you play a track put your fingers on the sub surround and you can notice it pumping in and out that is not in the music that is your feedback loop. (It is so low frequency that you don’t hear it and those frequencies is the one you cut out and stop the loop with the rumble filter..)
It is folly to spend a lot of money on any platform (many of them do not work) for such an inexpensive turntable. Buy a Sota Sapphire instead and all your problems will disappear. 
I do not think that any platform/turntable can make physics laws to disappear.

If as I guess most of us do not play TT and also have 3x15" subwoofers and are able to play far below 20 Hz..
That is little of a precondition here.. what I am talking about..

The goal in matching a specific cartridge and arm is to achieve a resonance in the 10 to 14Hz range. Some feel that limiting this range even further, to 9 to 11Hz, is beneficial in reducing the effects of resonance.

The following formula for calculating the resonant frequency of an arm/cartridge:

Resonant Frequency = 159/((M + CW + FW) * C).

I`m going to try and isolate the TT somehow, using acoustic foam of some sort.
Not going to ditch the table for another one that`s for sure, sorry.

As always, you guy`s come through with pretty good even great advice. 
Audio-Technica designed pneumatic insulators (AT-616) long time ago and this is still the best solution for turntables under 60kg weight. 
Here is a far less expensive, not to mention easier way. Take some rubber grommets and use however many between the wall and the turntable shelf/mount. Upon tightening the screws/bolts (what have you) to the wall, the grommets will compress and provide the needed absorption, and or vibration dampening. Rubber washers would work as well....
  "...Take some rubber grommets and use however many between the wall and the turntable shelf..."

I actually used dense rubber matting about 1/8" thick between the two bolts I used to attach the Rega rack onto the center "spine" of the equipment rack.
I wish Agon allowed pics...
I gotta say I agree with others who think that bolting the shelf to the rack ends up with transmission of energy from the rack.  But the OP is the only one who knows what that looks like.  Maybe there is something about the arrangement that mitigates what we see as a problem. However, the points about fastening a "wall shelf" to a weight bearing wall are correct, based on the received wisdom.  I do agree that concrete transmits energy, but I can't agree it "rings like a bell".  That metaphor calls forth the sound of a bell, metal on metal.  Concrete does not "ring" like that in my experience.  Also, in my house where I have two completely separate systems, one of which is in our finished basement sitting on an indoor/outdoor carpet over a solid cement foundation flooring and the other of which is in our living room on a suspended floor supported by studs (or whatever you call those wood support beams placed about 12 inches apart) and a steel beam as well, the basement system is far less susceptible to floor-borne feedback.  The two turntables in that system produce a very solid image. So, I don't think concrete flooring is to be sneezed at.  It's a good start.
The OP says he is using 3 woofers in this system and is planning on adding more.  Is this the Audiokinesis woofer system or just an attempt to wring out the lowest possible frequencies?  If the latter, that doesn't help with eliminating various forms of feedback.