Flap Howl
I work as c/c on the 146 and we do not mention the 'flap howl' to the pax at all. We never get anyone asking what it is either!
It is louder on the older style -200's, though they managed to calm it down a bit on the later-build -200's and 300's and is probably the same system incorporated on the RJ's. I believe they fitted shutter-like slats over the flap track which helped to reduce some of the noise.
The howl is useful to us c/c though because it gives us a rough indication of how far into the t/o /decent we are, it's also kind of a re-assuring noise when you are used to hearing it day in, day out!