With analog servos this USUALLY means your linkages are binding somewhere slightly and the servo is fighting to get back to neutral. Disconnect the linkages to verify. Then reconnect one side at a time and you should be able to decide WHICH (if not both) side is binding. Find the cause of the bind and remove it and the buzz should go away. Linkage could also be improperly bent too. Do the ailerons move freely when the linkage is NOT attached? I mean with NO drag at all near neutral?