About 20 years ago I was on a 90 minute Fokker F-27 flight on which for the first 30 minutes or so it sounded like someone was on the outside of the plane beating it with a hammer - a very large hammer. Later when the air smoothed out and the noise went away the flight attendent came around serving drinks. I asked about the hammering, and he said he was curious himself as he had never heard anything like it. A bit later he came back, told me he had checked with the pilot. The noise was ice breaking off the props and beating against the fuselage. Until then I hadn't realized the skin has to be tough enough to withstand this beating. I made this flight twice a week for 2 years an never again experienced the ice hammer.