THUD

[John Minor]

How about tightening all screws on the underside of the keybed?

I've found the jack can slap against the tongue of the hammer flange and
cause a strange noise. It can be reproduced by pushing up on the jack
with a snap as if you were attempting to block the hammer against the
string.

Also, I've had loose drop screws,(if you can turn them by hand it's too
loose) and had to shim them with small pieces of traveling tape.

Good luck.

John Minor
University of Illinois