A few days ago I asked PRIUS CHAT "What is the source of this sound". Since I posted this thread I have received many replies with different answers. I now think I can answer my original question. I stated when I open the Prius P driver side doorto get into the car, there is a sound under the engine compartment of a motor runnining - rrrrrrrrr" for about 9 seconds .
Ummm... okay, you said you discovered the answer, but it doesn't appear that you shared that information?
I wouldn't bet my own money, but I think I rememeber somebody saying that was the vacuum accumulator for the brakes.