Low Frequency rumble?

My guess is that you need to put your table on a more stable foundation...You must be picking up vibes from the floor/speakers .. etc.....I would consider mounting the table on a wall with a dedicated wall shelf ( Target wall shelf etc....).This will help elimnate the problems your having...If you can't do the wall shelf,you will have to provide a better environment for you current set up....

If you have a dustcover on yur table try taking it off and see if that helps.

My turtable is on a Mapleshade 'Samson v2' equipment rack. Very stable.
Elescher...yes, the tt does have a dustcover. I removed it, but the problem is still there. As soon as I turn it to a good volume level, the rumble begins.
Anybody else have any suggestions?

Try placing your TT on a piece of MDF or plywood supported by racket balls. This is a cheap test. Also make sure that the bearing is is lubricated properly and if the bearing type includes a ball bearing make sure that it is installed.