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After I've been driving for a longer trip, 30 minutes or more, the engine makes a weird buzzing noise for about 20 to 30 seconds after being turned off. It doesn't sound like a cooling fan. Anyone know what this is?
 

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coolant pump...

Yeah, it's a coolant pump. Cool thing, too. After turning off the hybrid system, the coolant is pumped into an insulated 'thermos'. There it sits, staying warm until you turn the hybrid system back on. Then it's pumped back into the engine block to warm the engine, in order to get it up to operating temperature quicker. Plus the heater should work faster too! It's all in the name of cleaner emissions...the mileage is just a bonus! Amazing car, lots more of these little 'invisible' engineering marvels, just keep reading these forums. Drive fun, drive clean, drive Prius! 8)
 
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coolant pump...

Yeah, it's a coolant pump. Cool thing, too. After turning off the hybrid system, the coolant is pumped into an insulated 'thermos'. There it sits, staying warm until you turn the hybrid system back on. Then it's pumped back into the engine block to warm the engine, in order to get it up to operating temperature quicker. Plus the heater should work faster too! It's all in the name of cleaner emissions...the mileage is just a bonus! Amazing car, lots more of these little 'invisible' engineering marvels, just keep reading these forums. Drive fun, drive clean, drive Prius! 8)
Correct me if I am wrong, but it seems to me un unnecessary complication with insignificant improvement for the intended use. First, is the whole coolant pumped into the thermos? it does not look like from the length of time the pump runs on my prius after turn off. Even if it was, I would be surprised of the warm/hot coolant from the thermos will be able to raise the engine block by more than 5 degrees, in hot climate or cold. Is that worth it? I am just very curious. Can someone give a more detailed technical explanation?
 
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