I second Niels Berkers' comments about a failed feeder bellows, as
well as yesterday's note by Glen Jedlicka [120125 MMDigest] about
loose legs and shaking.  In my experience, the loose leg is more likely
to be at fault, whether the glue joints have loosened or the attachment
means to the rest of the piano are failing.

Machine thread bolts into threaded inserts are the most robust, as
repeated disassembly/assembly with wood thread screws will eventually
lead to "overhauling" of the wood sockets.

A fast-response vacuum gauge should also help diagnose a failed (or
failing) feeder bellow as wild swings in the vacuum level should be
apparent.

But before you fix it, make a video of it playing "I Wish She Could
Shimmy Like My Sister Kate" and post to YouTube!

John Grant