As the video states, its the Power Transfer Unit, used to transfer hydraulic power between the Yellow and Green Hydraulic systems. On the ground you will usually hear it because the yellow electric pump is running and the green isn't. If one engine is running then a hydraulic system isn't powered so again you will hear it run. Most punters on hearing it think its a dog.
The PTU is like a 2 sided pump, the systems are hydraulically independent and it is only a transfer of motive force, not fluid. Hope that clears things up for you.