My "in warranty" PX780 developed a loud key (C2#). Many have reported fixing this problem by using canned air to clean the contacts. To fix mine, I taped the key cover open, tipped the piano forward to rest on the bench, and then glissed the keys a couple times. Problem solved. I suspect a small piece of tissue got inside as I had just damp wiped the keys. I did not want to use canned air as the one the store sold contained propane as a propellant, and I worried the propane would not be good for the sensors.
