Originally Posted by
Groundloop
I used to work with a BAe aerodynamics engineer who told me that they spent a LOT of wind tunnel time trying to cure the howl - but gave up in the end!
I was told that someone did eventually design a mod to reduce the flap howl, but only HM The Queen had it fitted - to "her" 146's !
Interestingly, speed brakes on 'modern' jets such as the A320 sometimes also produce a similar - but shorter duration - howl when they deploy at speed; going from high pitch to lower pitch as they open and vice versa.
Coming home to the UK from the south approaching London, there is a point where you are held above FL190, and then have to get down pretty steeply to below FL {I can't remember} within a few miles. This normally requires half or full speed brakes while doing > 250kts, which produces a howl that can sometimes be heard over London.