Looks alarming doesn't it? I've been coping with this phenomemon for decades and doubt it does any harm. I asked a trusted friend about it and he said all turntables exhibit woofer pumping to one degree or another. Perhaps Mara can address the electrical forces that make it move though it makes no sound.
I suppose you could construct a vacuum bubble or build a bunker for the table but let's keep it real. I had good luck raising my turntable about 18 inches above the tops of the speakers. It reduced the erratic woofer pumping quite a bit but did not eliminate it. Find a less sonically busy place to put your table. Or try the head in the sand approach and leave the grill covers on.
I suppose you could construct a vacuum bubble or build a bunker for the table but let's keep it real. I had good luck raising my turntable about 18 inches above the tops of the speakers. It reduced the erratic woofer pumping quite a bit but did not eliminate it. Find a less sonically busy place to put your table. Or try the head in the sand approach and leave the grill covers on.