Last week, I went out to my garage to investigate a motion sensor event that shouldn't have occured. It was about 3:00 in the morning. The Prius had been powered off for probably 8 hours or more. I suddenly heard a strange buzzing noise coming from the rear. Too concerned with security due to the motion sensor event, I didn't spend a lot of time trying to figure it out, but it continued the entire time I was in the garage. The next morning, when I came back in, it was fine. The noise sounded pump like. In fact, it was somewhat similar to the coolant/thermos pump noise that comes from the front of the car, but about 2 or 3 times the frequency, hence it sounded more like a buzz. I have no clue what this could be.
Five hours after the Prius is powered down, a pump runs to verify integrity of the fuel tank system. Perhaps this is what you heard.
Or maybe the Prius was getting a blow job from the lawn mower. That's probably what triggered the motion event. Seriously, though, Patrick has this one figured out. The fuel system checks for leaks after several hours of inactivity, which usually means the middle of the night. Tom
