Please see an extract from my post to a similar thread on this subject some time ago – use search ‘flap noise’.
The BAe146/Avro RJ flap howl originates from the flap wing gap (slot) along most of the wingspan. The source is aerodynamic. Most high lift wing / flap combinations have such a noise, often heard from the ground. You can just hear a similar noise in the cabin on Airbus A320 when sitting near the wing leading edge. The primary difference between the 146 and Airbus is that the A320 has a low wing at cabin floor level that masks the flap noise, but the 146/RJ has a high wing position above the cabin. The 146/RJ also has more lift – still the highest CLMAX for any civil aircraft.
The noise is very speed dependant, hardly noticeable if the flaps are retracted at Vfto or lowered at min manoeuvring speed, but you will get max howl when selecting flaps at flap limit speed.
Another other noise with flaps down is a mild drumming in the rear cabin; this was fixed by use of flap track shutters over the inboard flap track. If you get a drumming noise at approx 180 kts, 18 flap then get the shutters regreased. The howl and to a lesser extent depends on whether the aircraft has pannier tanks fitted or not.
I hope that the above helps with Capt Claret’s question on the intensity of the noise, but don’t forget that turning / increased ‘g’ will increase the noise, and that every aircraft has its own characteristics due to rigging, sealing strips, repairs, etc. There will always be some howl.
The different flap extension speeds are predominantly governed by aircraft variant (Max wt), but particularly by max zero fuel wt. Note that for the very heavy wt 146-300 / RJ100 aircraft the Min Man - Vfe gap closes quickly.