As Kford says, it's a 146 lowering its flaps as part of landing checks a few thousand feet above (I took in its route-proving in the mid-80s, the first time it was noticed outside BAe) but not one at a cruising altitude when flaps are retracted. I hear it all the time over us near Denmark Hill and see it on Flightradar. It is unnerving when you don't know what on earth it is!