What causes turntable rumble

My Rega P25 tt has begun to have a low level rumble. Never present before it is audible during quiet passages.
what causes this and how can I rid my tt of this annoyance?
Is the rumble 'all the time'.
Is the ruble just on some records.
Is the rumble there even if the platter is just turning, and NO LP is playing?
or, is it only when some?/all? LPs play?
Feedback can cause rumble. Have you moved anything around in the listening area since just before it started?
The motor can cause rumble, is the motor suspended? if so it might be loose.
Have you changed the turntable in ANY way? Moved it, changed the tracking force (even by accident?)
You may have just improved your setup so it is more noticable. Folks used to used 'rumble filters' on the RCAs from TTs all the time.
If you can't find the cause, don't worry, just get the inline filters.
Could be time to clean and re-lubricate the main bearing ;--) And I have found the best sh!t on the planet (in terms of blackest audio background) is van den Hul's spindle/bearing oil. It contains suspended particles of zirconium oxide which act like microscopic sized ball-bearings to keep the metal surfaces from making contact -- this is especially important in belt-driven TT's where the bearing is always being pulled to one side.
Nsgarch...A plain journal bearing does not have metal-to-metal contact when the shaft is turning. The turning shaft functions as a pump to keep a film of oil between the metal surfaces.

Air bearings work the same way.
A plain journal bearing does not have metal-to-metal contact when the shaft is turning. The turning shaft functions as a pump to keep a film of oil between the metal surfaces.
Not true unless (A.) The bearing material is of the bronze oil-impregnated type (not generally used in TT's because it's too soft to sustain the side thrust of belt drives) or (B.) The turning shaft is grooved to circulate the lubricant up to the top and then somehow bored out to return it to a sump at the bottom. A plain turning shaft doesn't pump anything -- and simple capillary action won't keep the surfaces apart.
Disagree with Nsgarch. Eldartford is correct. IF ANY lubicant is in the shaft (ie: not dry) it will remain 'properly' lubricated.. to the point at least that no chatter will occur. There is definitely not enough side thrust to 'wipe' the contact area dry. (Go ahead, try.. push as hard as you can sideways.. see if you can push that oil film out of the way (heh heh your gonna need thousands of pounds of side thrust..) In an automobile engine certainly, but not in a plain bearing TT.
Elizabeth, I think A.J. van den Hul and my mechanical engineering colleagues at MIT would strongly disagree with you ;--)

rumble is there on all records and only when a record is playing. had increased VTF as recommended on a prior thread because the cart didn't sound very good and someone suggested to increase VTF to max recommended to sound better. cart has 50 hours and sounds better probably due to more break-in, but with rumble. I'll drop the VTF back today and see/hear what happens
added a few drops of sewing machine oil to bearing which was almost dry, thought not completely
Nsgarch..."Wiffle grooves" are usually used on air bearings such as those in the gyros of the missile guidance systems I worked with. However, as Elizabeth says, regular journal bearings without grooves do pump oil quite nicely. Go look at any mechanical engineering text book.

By the way, your MIT connections don't impress me. Been there, done that.
Eldartford, I didn't know you were an alum! What year did you graduate, what course?

Wiffle grooves are not just a feature of air bearings (and BTW, many air bearings DO NOT use them, opting instead for multiple inlets in the manifold.) Grooves are also used on the Caliburn TT (I think) or some other expensive TT's that use a recirculating lubricant system.

But simple rotation alone creates no pumping action, and besides, what would be the benefit of "pumping action" if the bearing is filled with an adequate amount of lubricant which doesn't require cooling ;--) Metal to metal contact however is a problem. It is virtually assured in belt drive TT's; or whenever the tolerances are a little sloppy. No amount of lubricant will actually keep the metal surfaces from making contact, and that's among the causes of rumble noise.
Nsgarch...Although I attended MIT I transferred and graduated elsewhere in 1961. BMe. My work involved ongoing relationship (40+ years)with MIT Instrumentation Lab, now Draper lab.

The gyro wheel with air bearings is not driven by air jets, but rather by a motor. The wheel assembly is sealed.

It's been many years, but I seem to remember that the viscosity of the lubricant drags it into the narrowed part of the bearing clearance. As I said, check your text book (or maybe look in Google).
Colonel "Rusty" Draper, who for those who don't know, developed the Inertial Guidance System for our early guided missiles, was also the faculty advisor for the MIT Flying Club (which still blows my mind!) My favorite story (of his) was when he thought he was going to die, as he was flying a WWI 'Jenny' over Logan Airport, and the entire tail assembly fell off! (and he had no parachute ;--) He cut the engine, and hoped the wind would blow him toward Boston Harbor; instead, the airplane just "slowly maple-leafed to the ground" as he put it, "and I just walked away!" I often wonder (but never thought to ask him) if that's is where he got the idea for the inertial Guidance System!!

As for the TT bearings: if you suspect a little bearing noise is getting into your music ;--) please just try some of v.d.H's spindle oil, OK? Unlike so many what I call "Emperor's New Clothes" audio products, you will definitely hear an improvement in terms of quieter background, especially on old belt drive units. I know this because, besides my guinea-pig self (who of course sprung for a $50 three-lifetime-supply bottle of the stuff;--) I shared some with at least 4 others who said it made a real difference. Of course THEY got it for free, so I guess they wouldn't say it sucked!
Nsgarch...Doc Draper first developed Inertial Guidance for aircraft. The first US Navy Inertial guidance system, MK1, for the Polaris missile was actually a clone of a German design for the V2 which (fortunately) never got into production. Draper's main contribution was the floated Inertial Rate-Integrating Gyro, IRIG, both for attitude sensing and as a component within the Pendulous Integrating Gyro Accelerometer, PIGA. MIT Instrumentation Lab got the guidance system contract because at the time they had the best gyro expertise. I worked on Mk1 (a bit) and all subsequent systems including the next generation, not yet deployed.

I never heard him called "Rusty". Where did that come from? It was always "Doc".
"Doc" was what all the kids and faculty called him. I think the "Rusty" must have come from the people out at Hanscom Field, where he arranged for us to use the Link trainer . . . . a very colorful man.
I don't think sewing machine oil is thick enough.The factory says they assembled it with a special thick lubricant.The manual states that.Manual>>[http://www.rega.co.uk/downloads/P25_Man.pdf]