Hello, we've had the PX-850 for several years with no issues. Recently, whenever we hit a high E (two above middle C), as well as E-flat, there's a very loud electric buzzing noise in the left speaker. It sustains as long as we hold down the key. It happens in Concert and Modern Mode, but does not happen in Elec Piano mode. If we press the key very lightly, it does not happen. I've tried turning off and on the back-up feature in case that was the problem. I"ve also unplugged the piano and left it off for a while before plugging back in. Has anyone encountered this? Any tips?