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

Could also be FOPPs, or both combined! (Fuel Overpressure Protection) https://aviation.stackexchange.com/questions/23701/what-causes-descending-aircraft-to-make-a-noise-like-a-giant-blown-bottle 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.