I agree with JimmyTap - almost certainly the flap actuators, which are presumably mounted in the wing centre section and therefore effectively in the cabin roof. The noise sounds like screw jacks, and effectively seems to get louder as the accompanying aerodynamic noise lessens as the flaps go in. For landing the reverse is true, with both aerodynamic noise increasing and a rather pronounced pitch change as well.
That's one thing you can say about being a pax in a 146/RJ - you can at least see everything happening!