I am looking at buying a nice 2009 Prius that is a great deal, but while test driving it I noticed some strange things and I'm not sure whether to get it or not. This will be my first car and I have never even ridden in a hybrid or electric car, so I'm not familiar with what they are supposed to sound like. While I was driving, it made a repetitive whirring rattle that I can't find anywhere on the internet. The sound would last for about 2 seconds, and then stop for about 5 before happening again. It was always in these even intervals, and it didn't change based on how I was driving. It only happened while the car was running and was louder on the inside than the outside, so I'm thinking it's not for warning pedestrians. Under the hood, it seemed to be coming directly from back right corner of the engine, but I couldn't see anything amiss back there. It also continued while I was parked. The owner said that it has always made that noise and that Toyota said it was ok, but I'm not sure I trust either of them. Any ideas as to what the noise might be?