Dear Bob: I would suspect a loose case screw somewhere. Loose metal parts, such as hinges and locks, have been the usual source of such noises on pianos I have worked on in the past. Once spent a whole hour trying to elimenate a buzz I thought was coming from a string, only to have the pianist point out that it was coming from the lock! The heat may have caused the wood in the case to expand and contract, leaving a metal part or parts free to vibrate.Good luck, Patrick Poulson, RPT
This PTG archive page provided courtesy of Moy Piano Service, LLC