When you hear rattling or buzzing sound coming from your piano.
Firstly, Move any object that could be catching the vibrations. Check behind the piano, as well, to make sure nothing has fallen behind the piano. If you’re lucky, you’ll find the noise was only something leaning against the soundboard or otherwise picking up vibrations from the instrument.
Unfortunately, if you don’t have a solution after checking those possibilities. It’s vital that you contact a technician immediately to diagnose the problem so it does not worsen. Some likely problems will include cracked glue joints, loose bridge pins or loose screws.