Some years ago a friend mentioned an annoying high freq vibration in his C182, coming from down the back somewhere. He checked all his antennas, and everything.
I jumped in the back and off we went. Level off, pass 90 kt, bzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz yep. He said "hear that? It's coming from down the back."
Well it was in front of me in the back seat. Looking around what do I spy between me and him - an air vent up on the left roof. Closed the vent, noise gone. Opened the vent just so, noise back. Vanes in the vent were singing like a reed in a musical instrument.
That'll be $500 mate - cash or card?