A good one is if you have part of the seatbelt out the door. On takeoff it starts beating the side of the airplane silly and sounds a lot worse than it is. First time my buddy had done it, he figured it out and pulled it in. Second time it was my fault, taking my kid for a ride and got the same thing. Fortunately I figured out what it was and just gave a good tug on the loose end.
-- IFMU