This is a 1913 New York Steinway M. The sound on this video isn't especially representative, but is posted to help give a visual sense of how this noise occurs. This pedal clunk (@0:15) happens only at the end of this sequence:
Keys are struck
The sostenuto pedal is engaged
The same keys are released (their dampers remain up, sostenuto still engaged)
The damper pedal is engaged
New keys are struck
The sostenuto pedal is released
The damper pedal is released: CLUNK
Please note, the noise upon the release of the sostenuto at the 0:12 mark is just clumsy pedaling. The noise in question, at 0:15, happens midway through the released pedal travel and is accompanied by a slight plucking resistance, felt through the foot. Thanks for any help!
Hmm...Assuming this piano has the original damper system, it would have the old-fashioned fixed tab for the sostenuto. I don’t think you can fix this clunk, short of replacing the back action so that it has movable tabs. I just tried your sequence on our 1917 S&S model A, and I get the same clunk. You can also feel it in the key if you try to add a note to your collection of captured dampers—an unfortunate hiccup. By the way, you don’t have to add notes (as in step 5) to get the clunk. Simply engaging the damper pedal after capturing sostenuto tabs will create the problem. Good thing this design was modified eventually—a big improvement.