any ideas?

I have a Rythmik sub that I have been using for several years in my system.  It is connected via high level ( speaker cables from amp) interface.  It has always been quiet and actually a great sub.  For the past week or so, it has been making a mechanical buzz - NOT thru the speakers and not changing with volume.  Absolutely nothing in the system has been changed for several months nor have any changes been made in where things are plugged into.  Does anyone have any suggestions as to what might be the culprit?  The buzz  is annoying when no music playing, but the sub still seems to function perfectly when fed a signal.  I am at a loss..