I was reading posts "online" about this, and the answer seemed to be the quality/type of felt Casio uses for the key rebound damper. As a side note: I like to find out as much information I can about any big $ items I purchace, so I looked up the address/phone number for the service center in my area. I gave them a call to find out if they had been getting any 350's in for service. The "owner" said that he didn't know, as he had "service people" in back to do that work, but he said that he had just gotten a Service Update Memo from Casio regarding the 350. He read it for me. "....if you have any PX-350's come in, call Francis first". It didn't say anything else and I didn't ask who Francis was, but if a 350 came in, Francis was called first. ????? Wonder if there is a serial number range that Casio is looking for, to see if maybe it would be better to just send out a new one instead of fixing the old one. Who knows.