The "barking dog" - more like a pig grunting - is the hydraulic PTU (power transfer unit), as flyera343 has said. PTU is the Airbus term for what Boeing used to call a "reversible motor-pump" (RMP).
The A320 has two engine-driven ("ED") hydraulic pumps: one per engine. No 1 powers the Green hydraulics; No 2 the Yellow. (The other hydraulic system is Blue, powered only by an electric pump - or the RAT.) If only one engine is running, its hydraulic system will be pressurised by its ED pump. If certain conditions are met, however, the PTU will start running.
Take the example of No2 engine running, so the Yellow system is pressurised. Fluid from Yellow turns the PTU motor, which consequently runs the pump on the Green side, pressurising the Green system.
Regards,
Chris
The Yellow can also be pressurised by an AC (electric) pump, similar to the one on the Blue system. It makes a high-pitched whine.