Could be an engine intake noice. USN S-3 Viking's nickname was the 'Hoover' because it makes a very distinct noise when power is applied at low/medium power.
Used to play golf at NAS North Island in KSAN and they'd overfly the golf course at very low alititude - whooot, whoooot, whoot. You immediately realize how it got it's nickname.
A-10 and CRJ's have similar engines but different cowlings and the sound is completely different.
Some aircraft wing/flaps make distinct airflow noises as the flaps are extended/retracted. DC-9/MD-80's leading edge extension/retraction is an example. It's probably not loud enough to hear from the ground, especially 10,000-15,000'.