Yep, it is flap howl. Caused by the airflow resonating when the flap gap is a certain size, and changing in pitch as the gap size changes when the flaps are running in or out. A similar process to organ pipes, where air is blown across a gap to produce a note.
When I flew the 146, I was told that the Queen's Flight, (which used 146 or variants) had a modification to reduce the noise, but I can't imagine what that would be, or even if it was true.