One problem related to transmission noise has been reported for the 2007 Toyota Prius. The most recently reported issues are listed below.
Last week my wife was driving when our car lost power, began making unusual noises, and multiple warning lights became illuminated on the dash. She stopped the car, turned off the power, and restarted the engine; again the warning lights remained lit. We immediately took the car to our local dealer and left it overnight. The next day they informed me the transmission had failed and charged me $5400 to repair it. The day following the repair the "check engine" warning light came on again while driving and remained lit. We immediately returned to the dealer. After examining the vehicle the dealer told us that the water reservoir float had become stuck. He also stated that this was a common problem for hybrids which have undergone major engine repair. Today they further advised that we continue to drive the vehicle without further repairs.