knowing what the servo is connected to would certainly help us help you. If on throttle, as suggested already make sure the servo is not trying to overdrive the travel of the carb. If on a control surface I would suggest disconnecting the linkage to see if the buzz continues with no linkage. If it does either move the servo to throttle or replace it. I would prefer the latter. Under zero load an analog servo should be silent. If the buzz stops without linkage, check to make sure the linkage/pushrod moves freely. You could have a hinge binding issue or if it's a metal pushrod that runs through a tube, the pushrod could be binding in the tube. Applying some car wax to the metal pushrod will help that situation.