mfaff is quite correct. It is the changing airflow which creates the characteristic 'roar'. The flaps are driven by standard hydraulic motors which themselves produce almost no noise as flaps are deployed. To prove this, you will have heard no noise as you taxied out and flaps were selected prior to take-off, and again as the were selected up as you taxied back in.
The audible noise is only experienced during the first stage of flap retraction, or flap extension (ie selecting flap 18 to flap 0 after take-off, or during the initial approach when flaps are moved from 0 to 18 degrees).
The 146 is a fantastic machine to fly and produces many clunks, bumps, screams and roars during flight!!! I like to think that it is such a quiet machine, that it announces our impending arrival!
I used to wind people up by saying the noise acted as a bird scare technique during the early and final stages of the flight! Unfortunately, I keep being told that we only ever get birdstrikes from behind!
RM