question about noises under the hood just after shutdown

[M_W]

Hi, I have a 2005 Prius in generally good condition. Lately my car has been making an unusual noise, and I thought I'd ask the Internet what it might be before I try to face a mechanic.

First, some background: Sometimes when I park and shut off the car after having driven for ten minutes or so, I notice that something under the hood makes noise for a little while (maybe ten seconds? I haven't tried to time it)--most recently, I listened especially for this, and noticed that the sound actually began after I'd turned off the car, only to stop a little while later. Now, this isn't the problem that I'm concerned about--I don't care what my car does after I turn it off, as long as it's supposed to do it. But if somebody can tell me what this noise comes from, it would be helpful.

Given that background, here's the problem: Lately, this sound the car sometimes makes after shutdown has become... whiny. It'll start out sounding like a normal fan or motor or whatever, and then it'll suddenly become somewhat louder and much whinier, and then after a few seconds it'll turn back to the old sound. It goes back and forth like that until it stops. It sounds like maybe a belt is loose or a wheel needs lubrication or something like that. Can anybody suggest what the problem might be, and how worried I should be about it?

[stephen2002]

That's a pump transferring your warm engine coolant into a little thermos. Keeping a portion of the engine coolant hot helps reduce the warm-up time when you go start the car next.

Listen carefully the next time you turn the car on, you should hear the same little noise for the same amount of time that you hear it when you shut the car off. Right after it is finished pumping the engine starts.

As long as it is still running I wouldn't worry about it. I have an 07 and in the short year that I've had it the coolant transfer pump has recently become rather squeaky as the temperature has gone down. I'm waiting for it to have some sort of problem before I bring it in.

[maseace]

There is a TSB to have the pump replaced if it's squeaky.