Have you ever played a key and heard a clicking sound almost immediately after you hear the piano start to sound? This can be caused by one of a few things:
1. Loose flange screw
2. Loose hammer head
3. Loose hammer butt check dowel.
Something is loose and it needs to be corrected.
To check for loose hammer butt check dowels watch and follow the video. From detection to repair.
Good luck and keep your butts clean