A320 aerodynamic "whoop" sound
Living about the mid downwind right hand position at Auckland (close to position LOSGA) we tend to get quite a few A320's flying over coming in from the Tasman that are beginning to slow to join either the ILS or set up for a Visual Approach.
Often there is quite a loud whoop sound that starts with a low aerodynamic rumble but increases in pitch fairly strikingly.
Has anyone else noticed this or are there any NZ A320 pilots that might be able to clue me in on the cause of this?
I have two guesses, the speed brakes lowering at the bottom of descent or the first notch of flap being selected and the air rushing through the opening of the LE slats.
Any help appreciated.