Two problems related to loud engine noise have been reported for the 2010 Toyota Prius. The most recently reported issues are listed below.
The incident occurred when I attempted to accelerate onto a two-lane highway. Because of a slow and sharp u-turn which precedes the short on-ramp, I disabled the eco mode for better acceleration. When I pressed down on the gas, the car immediately emitted a loud sound, became unresponsive and was unable to gain additional speed. Even so, there was no other option but to merge onto the roadway while doing no more than 30 mph. In the meantime, the instrument panel showed a "check hybrid system" message along with several warning lights. I managed to avoid oncoming traffic (from behind) and safely pulled onto the grass. Attempts to restart the vehicle were unsuccessful with the previous message reappearing each time. The car required a tow to the nearest Toyota dealer. I received a call from them the next business day, and they explained the cause to be a defective inverter based on the diagnostic code. Fortunately, the repair will be covered under Toyota's hybrid warranty. It remains to be seen and I am hopeful this repair will fully resolve the issue.
The car was not used for about 4 days or so. When I started it, it exhibited a "rough engine start" for the first time. This happened when the gas engine initiated and lasted for about 10 seconds. During this time the engine made a very loud noise as if the car was chocking and engine wanted to get out of the car's hood. This has happened only once. The car stopped making the noise after 10seconds or so.