I just had a successful conclusion to a problem that has plagued my piano for 2 months and, for the most part, kept me off the keyboard for that length of time (by far the longest layoff I've had in 55 years of playing). On my Petroff, there is one key (F# about 2-1/2 octaves above middle C) whose mechanism is situated above the sostenuto bar but has no damper (it's the first key without a damper). In July, probably due to humidity, this key's mechanism hit the sostenuto bar when it was released. I let it go for a couple of weeks and then tried to investigate. When I had re-assembled the piano, 2 more keys were doing it. I couldn't risk playing, but could not set aside time for the technician due to schedule conflicts. He showed me that if the action was put in, but not quite all the way, the clicking stopped. But, of course, this is not a good long-term solution, so he filed down a few parts (whose name I don't know) and the collision stopped. We did not have a particularly hot summer, so I did not run the AC much. Unfortunately, that's the only dehumidifier I have. Before this happened, I had no idea that the sostenuto goes that high. I've never used it for anything other than bass pedal tones. Am I alone in this?? Anyways, I'm mucho rusty but I will now try to resume practicing, etc.