Martin Logan Descent makes “gyroscopic” sound


I’m about to take it apart but wonder if anyone had had and fixed this problem before.  
I’ve tried adjusting all the subwoofer settings and the sound that I can only describe as similar to a gyroscope or marbles at even one quarter volume. 
stevezee
Thank you.

It only happens at a quarter volume and up and not when there is no music playing 

I’ll check the coils. great idea. There are 3 woofers in a triangular pattern.

This has been going on for a while.
Ok problem solved. There was a large metal fastener stuck to one of the magnets of one of the 3 woofers. 

When the volume was turned up it most likely vibrated loudly.