Its nopt just the 319 it happens on. All the smaller buses have the same thing.
It is indeed the Hydraulic Pressure Transfer Unit (PTU) and mainly happens just as you go to start the second engine on push back. Although it does also happen sometimes on shutdown.
The noise after shutdown can also sometimes be the Yellow Hydraulic Electric pump kicking in to open the Cargo doors if the pressure left in the system after shutdown had dropped quickly.