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:
1. Keys are struck
2. The sostenuto pedal is engaged
3. The same keys are released (their dampers remain up, sostenuto still engaged)
4. The damper pedal is engaged
5. New keys are struck
6. The sostenuto pedal is released
7. 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!