Servos work by comparing two signals to each other. One signal comes from your Xmitter and is changed slightly when you move the stick. The other signal comes from the sevo itself and is altered as the sevo moves. A variable resistor or "pot" as they are called is mounted at the bottom of the servo motor. As the servo turns, this variable resistor is moved. THe signal in the servo is altered until it matches the signal from your Xmitter. When the two match again, the servo stops. It has a wiper and a coil, like a potentiometer. They wear out. The wiper oxidizes. The coil does also. The signal is sometimes lost and the sevo buzzes trying to align the two signals. Well, thats how old servos work. I don't know if they ever figured out a way to replace the mechanical pot with something else yet.
And as someone stated, standing very close to your airplane with the antenna fulling extended did sometimes cause chatter in the older radios. I haven't experienced that in a long time.


Make certain that you never route your antenna between your servos.