One problem related to engine noise has been reported for the 2012 Toyota Prius. The most recently reported issues are listed below.
My 2012 Prius v has several unresolved problems. On those occasions when the gas engine cranks the car, there is a loud engine rattler. The engine sounds as if there isn't any oil. Turning off the engine and re-crank does not always fix the problem. The front passenger seat sometimes rattle. Two different Toyota service departments are unable to determine and fix the problem. One of the shops replaced one of fifty button like leather seat fasteners. This seemed to resolve the problem. Two weeks later the problem started again. Now, I suspect the front passenger door speaker is the problem. Occasionally, the rear driver window squeaks. These problems have been reported to service shops but they want to keep the car over night. I did not buy a new car for over night sleepovers at the dealer. Finally, two weeks ago a rock hit my windshield. Five days later the windshield crack grew from a tiny ding to 15 inches. I have owned thirteen new cars in this life time. I have suffered windshield cracks but none of them suggested that the glass was too thin. This is my first Toyota.