Sometimes the cargo door operating handle can get stuck on, this results in the Yellow Hyd pump operating.
The whirring noise can be heard while walking around and can be heard from the flight deck.
Solution, waggle the door operating switch outside, or pull the cargo door CB on the FD, remembering to reset it before the handlers try to close the door.
We used to have to do this to avoid Yellow Hyd Overheats while on the ground.