A question for MMD piano techs:  I'm working on a Boesendorfer grand
which has a loud rattling or buzzing sound heard when playing 3 or 4
notes in the area two octaves above middle C.

After eliminating obvious sources such as screws, hinges, a paper clip
on the sound board, sympathetic vibration of an object in the room, etc.,
I crawled under the piano and had an assistant play the buzzing notes
continuously while I pressed on the bridges and sound board.  The noise
seemed to disappear or greatly diminish when I pressed on a particular
rib where there was a hairline crack running along the grain of the
sound board.  However, when I tried to stick a feeler gauge between the
rib and the sound board, there was no space to get the thinnest gauge in;
no separation between the rib and the sound board.

It's my understanding that a crack in the board by itself does not cause
buzzing; only if there is separation between the rib and the sound board.
Has anyone had any similar experience, and any advice to give?  Thanks in
advance.

Larry Lobel
Virtuoso Piano Service