One problem related to engine noise has been reported for the 2011 Toyota Prius. The most recently reported issues are listed below.
At approximately 120,000 miles we started receiving messages from our dealership that the car was starting to use oil even though we have performed all required maintenance and on time. Then between 130,000 and 140,000 we starting getting an oil light that would come on during cornering. We were told to keep a gallon of oil with us and fill it between changes and we did. It would use about 1 to 2 quarts between changes. Then at 150,000 the engine blew up. It was within the time limits of the warranty but not the mileage so Toyota would not help us out on a new motor. After that I researched on line and others are having similar problems. Most if not all our miles were highway cruising miles so easy miles. We drove this thing like a baby. We have owned many other vehicles to well over 250,000 so this is not a case of we don't take care of our vehicles. The motor right before it blew was making a loud screeching noise but no lights on on the dash to indicate that anything was wrong.