Same thing occurs on the 767.
It only happens though when selecting "Flap 1", which is leading edge motion only, so I presume then that this would be the forward PDU in operation. The noise is quite noticeable on the flightdeck.
The noise can be heard on the ground also. Can always tell when a 767 flying overhead selects the first stage of flap; it emits this strange "uuurrrrt" kind of sound
