[oe]

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.

[LargoLasskhyfv]

Could also be FOPPs, or both combined! (Fuel Overpressure Protection) https://aviation.stackexchange.com/questions/23701/what-caus...

Some do it at certain airspeeds, others don't. Depends on their shape, and can be mitigated/eliminated, which is sometimes done, other times none.