If you are listening to a record with the bass up (as shown in your pic) you will get more low frequency (subsonic) rumble from the turntable. Try turning on the low filter and see if the speaker moves less with very minor reduction in low frequency output.
You didn't mention if it only did it while listening to a record, but this is what I would suspect.
You didn't mention if it only did it while listening to a record, but this is what I would suspect.
