Rob
You did not specify what frequency or signal source.
After you and Danny have determined that it is not a internal driver issue.
I would start with checking the woofer bolts, after a while they need to be reasonably evenly re-torqued down.
Be careful, one slip and you need a new surround. ( don't ask why I use allen key bolts and socket nuts now ).
Next are the cabinet bits and bolts, if the sound is difficult to localize try the old mechanics stethoscope trick,
preferably hold the fat end of a long socket extension or in a pinch a big screwdriver to your temple,
carefully touch the other end to the hard! / solid! / non electrical! points of interest. You would be surprised what you can hear.
Shawn