Turntable rumble: what causes it?

Noticed in my second system that the mid/woofer's of my 2 way Joseph Audio Pulsar 2's sometimes appear to be quivering during vinyl playback.  I do not hear anything but I can SEE the driver's moving.  Does not happen with digital.

Is this evidence of "rumble"?

Wondering if the etiology is the turntable itself (maybe the bearing or motor noise) or whether floor or room vibration is reaching the table externally?  The turntable otherwise operates silently, as best I can tell.

My turntable is a Pioneer PL600 (vintage) on which is mounted a Hana EL cartridge.  Amplifier is a Pass XA 25.  Preamp is a Classe Audio DR 5 (which does not feature a rumble filter BTW).

There are long threads on "woofer pumping", and there are several possible causes.  You might do a search here and on Vinyl Asylum (go to Audio Asylum and select Vinyl Asylum from the list). VA has a good search engine.
"The turntable sits on an antique wrought iron table with glass top. It sits about 5 feet from the speakers. "
I saw this post from you after having submitted my last post.  At least by description, your set-up sounds like a set-up for woofer pumping.  Not only is it possible that your tt is too close to the speakers, but glass is not an optimal shelf material for a turntable, suspension or no suspension.  Glass is highly susceptible to environmental influences especially including airborne energy from the speakers.
The turntable sits on an antique wrought iron table with glass top.

Yikes! Won't hurt to lube the bearing. But ...  wrought iron... glass... there's your problem. 
it is possible that even the feet of the turntable, if they also have rubber, this has disintegrated and no longer acts as an isolator.

However, I believe that the Pioneer is completely underpowered compared to the rest of the system, even if the convenience of the automatisms it possesses can be indispensable.