What causes turntable rumble

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.
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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.