If the sound is changing at maximum deflection, the servo is probably trying to push the control surface further than it can move. With the EPA adjusted higher, I can definitely see this happening on a nexstar as they don't have a ton of allowable deflection in the ailerons.

Through normal uses, they will buzz though. They will buzz and center and they will buzz as they move but when the buzz changes there is a problem. It is so hard to describe this without some sort of demonstration. [:@]