In addition to what is mentioned above, if there is any real load on a servo while it is just setting there, because of something in the control system being bent or out of alignment, then that can cause a servo to buzz. Something like a pushrod being bent, binding in the hinges as mentioned above, etc. To illustrate this, while the radio is turned on, push on one of the control surfaces, and you will hear and see the servo fighting against the force that you apply to it. Something out of whack in your mechanical control system does the same thing.

If it keeps happening to whichever servo goes into that slot, then it is very likely something else besides the servo causing the problem.