Engine working forced!

How many miles on the car. Have you checked the oil level?

When the car is parked you usually can't hear the engine

 

welcome to priuschat! 2009 is not old, how are your mpg's?

 

Hi sadrac. Take a close look at the SOC (state of charge) bar graph next time this happens. In particular see if the SOC is maxed out at a the full 8 green bars when the engine does this.

 

Have you checked your exhaust system for leaks or holes?

 

loose heat shield?

 

Took the "how many miles on her?" question from you

When you are parked and engine not running, where is the noise coming from and what does it sound like?

 

try, 'when she is parked'.

 

I think that the OP is referring to the noise of the engine running at high RPM in circumstances when the engine wouldn't normally be running. Hence my previous question to sadrac regarding the state of charge while this is happening.

Sadrac, can you try to explain your problem a bit more clearly. Is it a case of the engine running (perhaps at moderate RPM) in cases where it normally wouldn't. This issue is sometimes caused by the HV battery having too higher state of charge, and the Prius consequently trying to dump some of this energy by spinning the engine without fuel injection.