AC electric pump to pressurise the Hydraulics (for the brakes), or possibly a PTU (power transfer unit) which is a way of transferring energy from one hydraulic system to another without fluid crossover (basically a turbine in the pressurised system drives a pump in the unpressurised system).
These can be quite noisy with one system depowered.
This is quite normal on pushback.
I'm not completly familiar with the Airbus family (Boeing driver myself), but either of the above would be a lightly explanation until someone who is A320/319 qualified can elaborate further...