We have planes pass overhead at about 6000ft. When the conditions are right they'll make a completely different sound, I've always assumed it's the Doppler effect mixed with the valley we live in but I'm always very curious when it does happen.

They make their usual sound but then there's a second sound that arrives, a lot higher pitched. Sounds like they've struck it in reverse or something (they haven't they're just doing a normal decent).

I think it’s some engine type that makes the sound at some specific speed / throttle setting. But I can’t remember the specifics. Some planes passing us make the sound, most don’t.

Very interesting question. I thought that was when they extend their flaps so the wings become a bit wider for the slower flight and that creates some resonance.

Extending flaps for approach makes a rather distinct woosh going from high to low pitch over a few seconds.

Sound is also reflected off a cloud layer if present.