Ah yes, the "Banshee wail". I almost sh*t myself when I first heard that doing LHR-STR on an LH ARJ85 back in '03.

In fact, I just left my house in SW London a couple of weeks ago and a 146/ARJ was coming in for LCY and I definitely heard the wail from the ground! So its not just an onboard noise.

I think it happens when the flaps come down, but beware if you've never been on one before because it can be a bit scary (especially down the back). Its slightly surprising that they don't make an announcement. I once heard (on a charter flight admittedly) the head FA make an announcement warning people to expect "increased engine noise after landing", i.e. reverse thrust!!! (not on a 146 though, they don't have reversers).