A4, the engines on the 319/320 always spool up on selection of flaps. But I do not think that's the cause of the howling noise.
It's either speedbrakes or the initial deployment of the slats. I think it is the slats. When you sit in row 2 or 3 there is a very distinct swoooooooooosh audible when the crew select F1 and I can easily imagine that this is very audible on the ground above and before the engine noise when and idle descent is flown.
But it could also be the speedbrakes since this howling sound on the ground sometimes changes pitch abruptly, which might coincide with selecting half speedbrakes.