I'm only a PPL, did I did ask my former neighbour (an A32x pilot) about this and IIRC it is a hydraulic transfer pump (PTU?), theat senses less pressure on one side of the circuits during engine start up and works hard to equalise it.
If I've got this wrong, please correct me!
He described it as a "clapped out mini starting on a frosty morning" sound, but dogs barking in the hold works for me too
