Originally Posted by
Shaggy Sheep Driver
. And I've never heard a noisier flap motor!
It's an aerodynamic flow howl between the flap inner edge and the fuselage as it passes certain settings. It was a surprise to the development engineers on the maiden flight. BAe did develop a mod to overcome it, but I believe it weighs 100kg (= 2 pax) and thus was hardly adopted, apart from on the RAF aircraft which form what was once known as the Royal Flight, which have it.
Cabin crew say it's a convenient prompt to start service on the climbout and give Cabin Secure on approach.