Hi Dave,
Ah, nostalgia for the days of vinyl and tweaking turntables! That's subsonic rumble and mechanical feedback that's causing your woofer "flap". No, it won't harm anything but it does waste lots of amplifier power that could be better used reproducing low bass frequencies. It also causes Doppler distortion of all frequencies reproduced by your woofer but there ain't space to explain that here....
Anyway, does your receiver/preamp have a (good) sharp rumble filter? That might entirely eliminate your problem. The offending subsonic garbage is typically between about 12 Hz and 20 Hz. If not, try mounting four old squishy tennis balls under the four corners of the turntable.
If the turntable has a floating, floppy sub-chassis suspension, like the old AR and Linn clone (Ariston and others used it as well), they were very susceptible to floor-conducted mechanical feedback in rooms with unsolid floors. You could try wall-mounting a shelf for the turntable to the wall studs. I've heard that is an effective solution. Never tried it myself.
There used to be dedicated silicone-type feet (maybe some discarded breast implants would work...) you could buy from Audio-Technica. Do a search for suppliers of turntable accessories. It's too early here for me to come up with the name of the outfit that supplies all that stuff. I'll post the name when I think of it.
Regards,
