Ron: Thank you for posting. I think your guess is right on! As you say, the thwack my husband and I heard likely had to do with the pedals and/or dampers.
Yesterday, before you posted your comments, I searched online for info on pedal noise and found this interesting Youtube video of a piano tech working under the belly of a piano to diagnose and fix a problem:
"Piano repair: Sustain pedal noise and its eradication"
In the video, if I'm understanding correctly, the tech identifies two different unwanted noises being caused by two different issues having to do with the pedal mechanism.
One of those two slightly different noises sounds just like the hard "thwacks" my husband and I remember hearing in the piano Webb was playing and also in one of the pianos I played during my piano search.
The particular noise I'm talking about can be heard in the video at 3:43 minutes. Also, at 6:16, it's the louder noise that is heard as the tech is saying, "Now we need to find out what this noise is…" - That's it! That's the noise!
The noise we heard at Webb's concert was much louder, however, than the pedal noises in the Youtube video; was easily heard over the sound of the Webb's piano playing and singing. I was wondering how the heck it could be that loud if it was an issue having to do with the pedals.
I was going to suggest in my reply (I prepared yesterday but failed to post in a timely manner) that maybe the audio set up at the concert somehow caused that. If, in fact, the sounds were being caused by the pedal mechanism, perhaps the location of the mics at the concert amplified the volume of that problem sound. But I waited to post my reply and just now see you beat me to the punch, came to the same conclusion as you say, "if a microphone was nearby, it would amplify the sound for all to hear."
Thank you for responding to my post, Ron. I believe you have solved the mystery and put this post to rest. I no longer have to wonder: "What the heck was that noise?"