string buzz quandry

[RustRazor@AOL.COM]

Hi all;

I've come upon a grand piano which has a note in the damperless zone (about 2 
1/2 octaves above middle C), that has a short buzz upon hammer impact.  No 
one string will buzz but any two out of the unison together will create the 
buzz.  If I take the butt end of a screwdriver and press on the strings where 
they cross the bridge, in between the fore and aft bridge pins the buzz goes 
away.

I don't have much experience with bridges and therefore ask any of the sagely 
contributors here for any suggestions as to how to deal with this noise.

Thanks, 

Matt Wynne, NY