Got home from work the other day, shut down my 2012 Prius 4 and went into the house. A couple of hours later went into the garage to drop a trash bag and heard a humming/whirling sound from the garage. Walked around the Prius and isolated it coming from the bottom rear of the car. Went and got key, turned the car on, then off again and the noise ceased. Anyone have an idea what that was? Fuel pump maybe? And why did this occur?
It's normal, it's the fuel evaporation leakage check and is mentioned in the manual... Enjoy your Prius Found this right here on Priuschat: http://priuschat.com/forums/gen-ii-prius-main-forum/73381-car-off-but-noise-coming-near-back-passenger-tire.html\
I had a '99 Jeep Cherokee that made the exact same noise, even a day after being parked. Never ran the battery down, and I never found out what it was....