As Max Angle said it's the Power Transfer Unit (PTU). Normal ops on pushback is to start the No. 2 engine on the right hand side first. This powers up the hydraulics on that engine and the system sees that the other hydraulic system has low pressure. It therefore powers up the PTU to pressurise the No. 1 engines system hydraulics. This runs up until the No. 1 Engine has been started and then cuts out. Common across all the A320 fleet.
Woof, Arf, Arf, Bl
dy loud if it's on during the walkround! (not to mention brake fans, APU and packs!! What??? Eh???? wheres me ear duffs!)