On the subject of 146/RJ flap noise.
It occurs with both the up and down selection of flap 18, No one has given me the definitive answer as to what actually causes the noise, but the best I have heard is simply that the change of airflow through and around the flaps causes it.
There is no such noise when other stages of flap are selected and it is exactly the same systems that are in use.
Still I agree it makes one hell of a noise and I believe that you CAN here it on the ground!