This Epiphone archtop came in for a setup along with a full fret level.  The fixed-top archtop bridge also needed to be properly compensated to keep the flatwound strings playing in tune.  During the final setup I found this annoying B note sympathetic vibration that was tracked down to the bridge tone cap physically vibrating against the pot.  This was a first for me!  Just glad I figured it out as rattles can drive you insane.