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.

How close is your turntable to your speakers? You might be getting acoustic feedback.

Very cheap and fast isolation platform/plinth. Go to a Home Depot or like. They have a pre-cut section of wood products. Look for what appears to be unfinished food preparation cutting boards. Then find drywall self tapping screws, the longer the better. Send the screws down through the corners of the cutting board, keep away from the outer edges of the board to prevent breakage. Before placing put a penny or something to protect the surface below. The drywall screws can adjusted for height leveling.
Also try moving the TT away from magnetic field producing equipment. Check the manufacturer’s web page FAQ, if any for set up location issues.

Good luck