"Pop" Noise from Thermos?

Ok, I have looked through the Inverter Pump Coolant threads trying to find an answer to this and am not having any luck so I started a new thread. I am not even sure this is a pump coolant/thermos issue so don't let the thread title throw you off.

Anyway, my wife's 2007 Touring makes an odd "pop" sound anywhere from 2-7 minutes after being started and driven. She described the sound as someone popping a plastic bag but I don't think it is that loud. It sounds more like the lid of a new jar of pickles or something else "sealed" being opened.

The sound only occurs once and will never occur again until after the car has been turned off and allowed to sit for some time. The shortest time I have tested is about 20 minutes. I haven't tried to turn it off sit for 2-3 minutes and turn back on to see if it happens again but probably will when I can.

Her car only started doing this in the last week when it has gotten unusally cold her in North Carolina for us, it was 22 at 6:30 this morning here. So that is what leads me to believe it is a thermos/coolant issue related to cold temps. Maybe something in the transfer is under vacuum that is being released or maybe something with the "shock" of hot/cold coolant is inducing the "pop" effect?

These are PURE guesses on my part so if anyone has any ideas please let me know. Or if anyone else has heard this type of noise in their Prius feel free to chime in. My 2006 Standard has never made such a noise and it has 134K on it now. Hers has less than 30K.

Thanks in advance for all the help. PriusChat rocks!!!!

Chris

 

Ok, here is some additional information. As a test my wife's car has been parked in the garage overnight the past two nights with a temp of approximately 50 degrees F as a low overnight in the garage.

When she goes to work in the morning the outside temp has been in the upper 20's low 30's so really cold still for North Carolina. She has not experienced the "Pop" noise on either day. This seems to indicate the noise is most definately temperature related.

The next question would be is when is the "thermos" filled with hot fluid? Is it at the end of a long drive when you shut the car off or is at filled as soon as the coolant reaches a certain temp?

If it is when the coolant reaches a certain temp could this account for the "Pop" and explain the varying times when it occurs? If one day the coolant comes up to this temp in 2 minutes the "pop" occurs then, if on other days it takes 5 or 6 minutes to reach this temp it occurs then.

Just thinking about the noise and trying to find an explanation for it. Anyone out there with more insight and knowledge on the "thermos" circuit please feel free to respond.

Thanks,
Chris

 

Tom,

Thanks for the idea I appreciate the response and will see if I can determine if it is something expanding with the heat and then contracting with the cold.

I was just thinking with the extremely repetitive nature of the "Pop" that it was more likely something mechanical that occurs in the 2 - 6 minute window after startup. Especially with the vacuum release sound that it seems to be making.

Thanks again,
Chris