This could be it. The coil on the contactor or relay can sometimes buzz if the contacts don't meet just right. Keep an eye on it. It could be a fluke or it could be that the contactor or relay might be showing it's age.An AC relay can occasionally buzz somewhat if it pulls in for some reason just a little out of its normal position. My Clipper Creek has not done that (yet), but if it does I would not worry unless it becomes regular.