When I was a kid, there used to be a military airfield near where we lived, that had normal helicopters and tandem-rotor helicopters that they flew by with daily. The noise was quite distinct.

Now we have ambulance helicopters and police helicopters flying directly overhead from twice a week to twice daily. I stopped looking years ago until I one day heard a different sound, yet oddly familiar somehow, that sounded like something was punching the air aggressively with fists. I had to go out and saw that ancient dark green monstrosity of a tandem-rotor helicopter again. And then another. Last year they seemed to have daily exercises for a week with them during defense training operations. Didn’t want to complain about them online since it would be inappropriate to share pinpoint locations and times, but was very grateful when they stopped.