While I have not flown this type, I do remember a highly distinctive wailing sound the flaps emit when travelling through a certain setting from my flights as pax. As You have heard this during acceleration, this might be the cause. In fact, one of the many nicknames the 146 bore was "Hound of Hatfield" due to this noise...
EDIT:
Starting from 0:39 in
, You can hear the sound I was referring to.