Flaps
I suspect they're lowering flaps.
The Bae146 made a quite spectacular sound due to increased drag when flaps were lowered, clearly audible inside cabin, to the point that many airlines would incorporate mentioning it in the cabin crew PA announcement on approach... basically, "don't worry, the sound is normal, the plane is not falling apart."
I live under the approach to AKL, and the sound of increased drag as flaps are lowered is often the first thing to wake me at 5am, as NZ1 arrives from LHR.