Turntable Thrumming

I'm a new member and have been following the forum for a while.  It's a pleasure to read the learned insights of the members and get information on a wide range of subject matter.

I recently bought a Pro-Ject X2B turntable and am generally pleased with its performance.  But I've noticed a low thrumming sort of sound.  Not really a hum or a rumble, just a low frequency low cyclic rate sound.  It's not very loud and only discernible during quieter passages of music or between tracks.  The turntable sits on a very heavy acacia wood table.  Any thoughts on the source of this sound?  Also, any advice on record weights and better feet for this turntable?

Thanks,

John Cotner

New Ulm, MN