Loud clicking noise on cold start

The (US) prius has a "Thermos" where it stores hot coolant when you turn it off. The thermos is located inside the left front fender.

It can keep the coolant hot for up to three days.

When you start the car, the hot coolant is pumped over the top of the engine block to 'pre-heat' it, shorten warm-up time, and decrease the amount of time the system has to run with a rich air/fuel mix.

This is part of the emissions system.

That's the sound you're hearing. If you sit in the car after you turn it off, you can also hear the thermos being filled.

There is also a little buzzing that sounds like it's coming from under the steering column that you can hear as you drive when it's quiet. That's the brake fluid pressure pump.

Some people report that the coolant pump has failed on them. The symptoms are that it just gets louder and louder over time.

It's covered under the emissions warrantee because it's an emissions related part, so you have at leasts 8Y/100K (or more depending on your location) before you'd have to worry about shelling out for it.

There may be pictures of the thermos in the files section here, do not know.

 

Tempus is right about the US thermos and recirculating hot coolant, but I've never heard it "click". I hear sort of a buzz as the pump moves fluid from the thermos to the engine.

If you're hearing a strong clicking, it may be something else, though I don't know what. This is the kind of thing where you'd need to leave your Prius overnight at the dealer's, and ask that their Prius tech start it in the morning when cold.

Though I might not trust the dealer you've taken it to...

 

Welcome to priuschat jsikora. I've been hearing the coolant pump and the brake fluid pump since I first brought the car home. Could it have been doing it all along and you just didn't notice? Anyway you might consider putting your location in your profile. That way if any one else lives in your area they might suggest a dealer with a good Prius tech, also some questions are weather or climate related.