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After I shutdown my 04 Prius, I can hear a "buzzing" sound coming from under the hood. The sound lasts 10 to 15 seconds. What is taking place during this time and is it normal?
 

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Normal; I believe this is a pump pulling coolant into a "thermos bottle" to save it, and returns it back to the cooling system on the next start. It helps to preserve the heat so that it "warms up" faster the next time.
 
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