Although not entirely musically useful for me - the Rd/Clav/etc galleries with a scene which is of the instrument OFF and only unamped tine/key/etc sounds was an inspiration point.

All uprights I've played had a pretty noisy sustain pedal with a distinct thumping sound.

I pulled out my sustain pedal (normally I don't use one ... ) and checked out the upright sounds/etc. I'm not sure if the instrument can even apply a sound with press/release of the sustain pedal. What I found was none of the pianos have this extra noise that I could tell. No issue at all - this was out of curiosity.

That's how this started - but then I started to wonder if the Piano sustain would respond like a piano when "hitting/stabbing" a loud chord with the sustain pedal pressed, sustain still pressed and no keys pressed, and softly pressing one of the previously sustained notes - then releasing the sustain pedal. The piano would respond with no change in volume for the note assuming the note was hit softly enough as to not even hit the hammer - just enough to ensure the damper was still lifted when the sustain is "let go". However, the note is always made very soft when I let off the sustain. If I hold the note - the note is always louder.

While sustain is held and isolating to a single note, I noticed that if I re-struck the same key with the sustain - if I pressed softly enough the volume of the note did not increase or decrease (stayed the same) which is what I would expect as the correct response. It's when I let go of the sustain that the note (while still holding the 2nd soft-attack pressed note) was suddenly much softer (lower volume) when the sustain pedal was released.

The attention to key clicks on an electric piano or clav led me through this journey.