Flaps and/or slats being extended.
As mentioned above, the 146 flaps positively howl when travelling from 0 to 18 or vice versa. So much so that at the co I work for we mention it in the safety brief pa.
The Douglas/Boeing 717 that I now fly, also has a mini-howl as the slats extend or retract, but NO WHERE near as loud as the 146.
