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).
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