Very hard to describe this sound but it is coming from under the hood and lasts for about 10 seconds.
Is it a whirring or humming kind of sound? Mine does that, too. I figured it has something to do with the electric motor shutting down.
Perhaps you are referring to the sound of the pump on the coolant heat storage tank? After you shut off the car, this pump fills the thermos-like tank with hot coolant so that the engine can warm up faster and have lower emissions on the next start.
If you peruse the Owner's Manual, you will see several entries concerning various sounds that the Prius makes, when the engine is not running/Power is not "ON". All of these entries note that "this sound is perfectly normal"
It could be the sound of the coolant transfer pump failing. A failing one tends to get noisier in cold weather.
It is located near the coolant heat recovery system canister, in the front left wheel well in front of the tire. Replacement could be DIY if you consult techinfo.toyota.com or look up my post about how to replace engine coolant. However I wouldn't go to the trouble of replacing the pump just because it is noisy. If no DTC is logged by the engine ECU (which would cause the check engine light to appear), then no worries.
I have a 2007 Prius with a similar noise that seems to be getting louder/more obnoxious. Did the pump ultimately fail regarding the original post on this thread?
Maybe not, I have a 06 prius. the Inverter Pump get relative gentle noise in summer and loud in winter. I tried to bleed the pump. Yes, few air seems like comes out but it doesn't help about the noise issue. Never mind, I just ignore it. I will change the coolant in next summer regards to the 100K mileage.
