One problem related to engine noise has been reported for the 2006 Toyota Prius. The most recently reported issues are listed below.
I have a Toyota Prius (hybrid). A little cat got underneath my car without me noticing it and since this type of car does not turn the engine when you turn the car on, it does not make any noise (when you turn the car on it runs from the battery at the beginning. . . Eventually the engine kicks in) since the car made no noise the little cat did not notice that the car had been turned on and the little cat did not move away from the car. The little cat died within seconds and I had to watch him die, I had to watch him take his last breath, I hat to watch him suffer trying to get up I had to watch him wag his tail for the last time without me being able to do anything about it. This car is far too quite and dangerous for people and animals, neither one of them hear the car when you drive up right behind them and very often I have to watch out for people in parking lots because they never hear the car coming. The death of this little cat was devastating to me, I cried for an entire week and I do not wish that experience to anyone. I want to report this issue to avoid any other deaths of innocent animals and people.