engineer logic

My brother-in-law, a pilot, related this incident between a fellow pilot and an engineer/mechanic at the airport. These two chaps were forever squabbling and trying to drive the other one nuts. The pilot once turned a plane in to the shop with a complaint that read "Unfamiliar noise from engine." The next day, the plane was ready for him. The engineer's logbook entry read "Ran engine continuously for four hours. Noise now familiar."