Also, in regards to the OP, I have used "plasticine" (regular old kid's modeling clay) on a part that was singing (very loud), and it worked like a charm.  The piece was a 6.5" diameter x 1/2" thich brass plate that I was trying to cut down for a flywheel.  Since I have a small lathe, cutting from the outside was impossible, so I used a trepanning tool, and cut right into the face at around 4" diameter.  It was cutting, but making a terrible racket, and the finish was horrible. 

A glob of clay, rolled into a long "snake", and pressed on around the perimeter quieted it right down, and helped the finish as well.  Alcohol cleans off any residue left on the part, but any degreaser would probably suffice.  I got this tip from Bogstandard on one of the other machining forums.