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.
Funny you should mention that. Years ago, when the plane was on rental, a renter wrote in the tech log in Big Capitals "LOUD BANGING NOISE IN FLIGHT". Unfortunately we were not quite cute enough to make him pay for a full respray, though he did pay to have the dinks made by the seatbelt clasp in the outside skin repaired. We still have to put up with the slightly different shade of paint behind the door!