The noise may be caused by an exhaust valve which opens when aircraft are descending and the pressure differential reaches a low level. You can also hear it in reverse when ascending. You can hear it clearly whilst on board an MD80 as it climbs. I think this exhaust port is open on the ground and it closes as the pressure differential increases with altitude.On the way down it opens as the pressure differential reduces to a low level. In flight at altitude it is normally closed. As far as I understand it's an exhaust that dissipates warm air from the electronics bay.