In Digest 970928 Roger Waring writes:

> I found, however, that by lifting the strings fractionally off the
> bridge where they pass the pins the problem disappears.

A split in the soundboard is usually an indication that the board is
loose from a rib or ribs in the area of the split, and setting the board
in motion causes the board to slap against the ribs, causing the buzz.
In many cases it is only noticeable on a few notes.

Pressing on the board or the bridge may cause the buzz to go away
temporarily, since this alters the space between board and rib.
I suspect that when you lift up on the string, you get increased
down bearing on the bridge, causing the buzz to disappear for a time.

Bill Maxim