Had a friend who's helmet strap was flapping in the wind, clicking against the the little plastic strap adjuster thing. Drove him insane trying to figure out the high speed rattle on his bike.
My father once bought a Volvo, couldn't figure out a certain flowery smell he kept getting, thought there was some kind of air freshener jammed under the dash or something. Turned out it was the hand lotion he used to put on his hands sometimes at red lights.
-Tim-