How to eliminate TT feedback/vibration

1st Thing I will tell you - I have an OLD TT. It's a Pioneer PL-71. ( I'm looking at a Clearaudio CMB ) My TT is placed out in front and to the left of my left speaker(per Jim Smith's book.) I made a platform of MDF attached to the wall where my TT sits. At no point does the platform contact the floor. When I play certain albums-several in fact, at mid to high db levels, I get either an acoustical feedback or vibration feedback from low hz. I have tried to isolate my TT using something like sorbuthane balls and foam. I have not tried any of the commercial cone devices advertised. (Do they really work and if so-what's the best product?-no salesmen please). I don't know if my problem lies with my TT, or my platform it's sitting on. I have a friend who actually hung his TT from the ceiling years ago, and I may have to do the same. Thought I would inquire if anyone knew the answer before I started experimenting. Will a higher end TT like the Clearaudio cure my problem? Is it the platform I made-or do I simply need some isolation cones?
You need a rumble filter!
For inexpensive try - get 4 Vibrapods and 4 Vibracones, these cured my probelm.
I don't think you need to waste money on more "vibration control" crap, since you've already gone to the trouble of making a wall shelf. It won't do anything to reduce "airborne" vibration picked up by the table/arm/cartridge. I agree with Amandarae. You either need a rumble filter or need to move the TT further away from the speakers.
Better yet, try a set of 'moon gel' pads. Very inexpensive at $6. You can find them at most musicians stores. They are used by drummers to stop vibration. The pad's color is blue, each pad about one inch square. I've used these under tables and speakers.
A sub sonic filter my be needed. As for vibration control "crap", cones could be of great help. I have seen them help in the same situation. I'm not recommending spending a fortune on exotic stuff. You might want to try a set of Dayton dss4 speaker spikes from Parts-Express as feet for your TT. They are really just big brass cones. They are two piece for leveling, but I would screw them tightly together. Use 3 or 4 of them with a tiny bit of adhesive to keep them in place. Worth a try for $15.
Another thing you might consider is removing your dust cover and just placing it over the TT when not in use. The cover seems to pick up a lot of vibration.
Two things you could also try. You could mount the shelf to an exterior wall it should be less resonate and if you are feeling handy you could build a TNT sandblaster both have worked quite well for me.
What device do you recommend for a rumble filter? Of course, some phono stages have it built in, but if the one you own doesn't, what do you recommend?
Are you sure your cartridge/arm interface is correct? What cartridge are you using?
What exactly is your wall shelf design- no offense.

Otherwise maybe just change where it is at on the wall a little as it may be at a resonance frequency. A photo would help a lot.
I had the same problem with my VPI Scout turntable. I tried everything, then bought a KAB rumble filter. It worked like magic and did not subtract any bass from the music. There is a used one here in the classifieds for 99.00. I paid 169.00 plus shipping for mine, new. You insert the filter through the tape loop and when you play records, put the tape monitor to tape or it can be installed between the pre and power amp.

If possible, have the wall shelf attached to an outside wall or strucural inside wall as mentioned(behind the speaker plane). Try to set up the shelf so that it is not rigidly coupled to the wall, don't over tighten the screws and perhaps put rubber washers as spacers behind brackets at wall. Then decouple the table from the shelf surface by putting a seperate MDF board or buthcher block on it seperating it with cones,vibrapods,small spikes,rubber/plastic pads etc. Take the lid completely off while playing(antenna for airborn vibrations) also mentioned. Give the cartridge bolts a good tightening and make sure the tonearm bearing is not loose. If this don't help I would say you need a better table.
Me, I will build one (OP amps base) if I need one. There's a lot of circuits floating on the net somewhere. It is much cheaper to DIY.

If you do not want to deal with the hassle, just buy one (KABUSA carries them just like Bob already posted). Or for the best solution, get a new preamp that has one(i.e. Cambridge Audio 640P for a cheap alternative).

Of course, the cheapest way is to move the TT further away from main speakers, hoping that it will solve the problem.
It may be the turntable. I had an old PL-50 years ago and it had an aluminum platter that rung like a bell. If you could borrow a different turntable and set it up on the same shelf at least you could narrow down the problem.
If possible, try lowering the stylus on a disc without the platter turning, and then rapping on the wall, platter, plinth, platform etc with volume at realistic loud levels to get some insight into what is happening.

This may help you avoid less-promising approaches, and more quickly evaluate potential cures.

Keep in mind also that although a rumble filter will probably help, ideally you would eliminate the problem at it's source (assuming that's feasible), since your cartridge and phono stage are processing that rumble even when you filter it out.
I will try to get a pic of my set-up on line. I actually improved my problem by spiking my TT on a piece of MDF and put the MDF on some foam rubber. I can now increase my volume several db without any feedback and the spikes are responsible. What I need is some type of jell material I can place under the MDF board. Can anyone think of a soft jell material I can put under the MDF board? You know I had a friend that hung his TT from the ceiling and it worked great. I may try that too. Thinking as someone suggested part of the problem may be in the actual TT construction. I can peck on the TT at high volumes and hear it. As I mentioned, going to audition a Clearaudio CMB Friday. Guess I will see then. As far as filters go, I already have a rumble filter on my cheapo Balleri VP130. I kid you not, for the $$ that phono stage is well worth the bucks. Thinking about going to the YS Concerto Plus. I don't want to invest bookoo bucks at this time. Actually read a review where the YS won out over the Audio Accoustics PH3. Anyway-I'm making progress on my feedback problem. Will keep you all updated. Thanks for all the ideas.
Here are various Sorbothane devices. One of them may work for you:
11-22-10: Auxetophone
If possible, try lowering the stylus on a disc without the platter turning, and then rapping on the wall, platter, plinth, platform etc with volume at realistic loud levels to get some insight into what is happening.

Although I appreciate the reasoning, rapping on the platter (and perhaps even some of the others)sounds a dangerous proposition as the platter can easily be disturbed in the wrong direction leaving the owner with an expensive headache....
I'm not even sure that previously securing the platter gives the required degree of confidence. Very much done "at risk".
Minus K isolation works great for me.