Most likely , what he heard was the H-46 Chinooks.
CH-47 Chinook is a twin-engine, tandem rotor heavy-lift helicopter, much harder to “Stealth” one of those ....
Duh, I was in the air force for four years!!! I know what they look and sound like. Every effort to make them quite has a multiplying effect on loss of horse power and lift. In short they make the noise because they need to.