2010 Prius-Sounds Like a Fan Running

evap check, normal

 

the fan is somewhere toward the rear, the evap is part of the pollution control system. sorry, that's all i know

 

There is a whole "evaporative emission control system" that is there to make sure the vapor that evaporates from your fuel is kept in the car and used in the engine, instead of escaping into the air. Most of the related parts are under the body in the rear near the fuel tank, though some are also under the hood. Every so often, when the car has been off for a while, it will do a self test to verify that it is still airtight. There's a little vacuum/pressure pump back near the fuel tank that does the test. If the pressure doesn't hold, you will get a warning on your dash and a code about a fuel system leak. If you don't, everything is good.

I've never heard mine in action, but you can read all about it in the manual.

-Chap

 

Yaesu, as chapman said this sounds like the evaporative emissions pump running to test the integrity of the evaporative emissions system. I have encountered this several times going into the garage at times after the car was parked for several hours. I believe if you open the drivers door it will cancel the test, one way to know if this is indeed what you are hearing. The pump is part of the evaporative canister that is mounted under the vehicle just behind the fuel tank. This test is ran after the vehicle has sat for X number of hours, the fuel tank level is between X amount and X amount, the ambient temperature is above X degrees and below X degrees. The X has to do with parameters that I do not know, but must be met for the test to be ran.

 

...and for anyone wanting to catch this evap self-check in the act, it happens at ~5 hours post shutdown. Put your head down by the driver side rear wheel well and you'll be a believer. I'm a garage tinkerer and have heard this pump buzz for it's several minute cycle many times on both my deceased 2010 and now my 2014. It's an odd thing for sure when you first experience it.