 FACT: When the troopers lead by the Colonel stops no sound is heard,  all of a sudden you hear the sound of a helicopters rotor fade in, a  few seconds later the helicopter appears and starts to slaughter the  men. The night was clear and it was in a desert, those are ideal  conditions for sound to travel (even a whisper will be heard over a  good distance) and seeing how a helicopter creates a very high sound  from both the motor and the rotorblades they would have heard the  helicopter several minutes in advance.