In films I've always found it's the sound of a Bell 47, particularly when they are trying to start a Jetranger! If my engine sounded like that I'd take it straight back! I put it down to ignorance.
BTW, There's a film out called Supreme Sanction which has lifted the helicopter scenes straight out of Firebirds (or Wings of the Apache, depending where you are). All they did was replace the facial scenes in the cockpit with new people and write an even worse script.
Phil