I'm borrowing a friend's Fender Bassman 400 combo amp, that I maybe buy off him for $300 or so, and I've been using it as a practice amp only and its working out great except for this high pitched squeal that will begin seemingly randomly. I've tried turning the tweeter down, messing with the EQ, it even sounds while the Mute button is on. This combo amp has been in the shop before. For what I'm not sure. Does anyone know a solution to what the problem might be? Can you help me pinpoint it? If I can fix the problem without taking it into the shop, I'll most likely buy it.