When I lived in Tehran back in 1975-76 my apartment was situated very high on a hill about ten to twelve miles from Mehrabad the main airport. There was a difference in about 2-3000 feet in elevation between my apartment and the airport. We were on the main flight path between the Shahs’ flight and the palace. Any time a 205 or a 206 lifted off more than fifty feet and started heading for the palace you could hear it coming. It absolutely amazes me that they didn’t lose more of these helicopters during the Vietnam conflict because of the telegraphing of the helicopters’ rotor noise. The CH-47 is just as bad.