Have a 2006 S80....noticed that there is constant noise coming from behind the climate control module, so pulled it out to take a look. Found that there is a small fan (about 1.5 inch in diameter) installed to the back of the CCM, and that it runs continuously, even with the car turned off. I have never noticed this before, and don't think it should be running all the time, as it does not shut off. I asked a local repair guy, and he kept telling me that I heard the aux fan that can run for a while after turning the car off. Definately not the same fan I am talking about.

Anyone have any experience here, or know when this fan should/should not run?

Hi, I'm new to the forum and lurking to find what XC90 concerns are out there and I saw your post. I have seen the same thing happen with our V70 a couple times. Scenario is like this... its happened 2 or 3 times and I think its always been on very hot days, when the aircon has been on and the climate control has been set to AUTO. The wife parks the car without shutting the aircon off and eventually I come outside and I hear a fan funning somewhere. I check, and there is a fan in the dash or maybe the actual blower fan running for a long time and doesn't shut off. (not the rad fan, and one time it was running for more than 30mins) I get the key, turn the ignition switch on/off, and maybe turn off the auto climate too, and it stops. It never does it with me driving as I tend to like the air switches set to foot and face level (not auto). We did have a climate control computer replaced under warranty when the heat stopped working one winter ($1800... thankyou warranty) but I think it has happened before and after that was done.
Give it a try and let us know.

The small fan inside the ccm draws air in from the cabin to test the temperature. I had a very noisy one on my car and replaced it myself - it's not difficult, just an expensive part.