Just as the second engine (engine 1) is starting, but before it reaches idle, the hydraulic system goes through a test. The PTU is tested by using the Yellow system power (powered by the engine 2 hydraulic pump) to power the Green system before engine 1 reaches idle.
When engine 1 reaches idle, then the Green system is powered by the engine driven pump, and the PTU shuts down.
The sound you hear when the cargo doors are closing, is the auxiliary Yellow pump. The cargo doors are Yellow hydraulic powered, and this auxiliary yellow pump, (like the PTU) is located under the cabin, mid fuselage. When a cargo door is opened or closed, you will hear this pump start/stop.
Just because I stopped arguing, doesn't mean I think you are right. It just means I gave up!