Noise from front driver's area even with car off
 

My 2002 S80 T6 will occasionally have a noise that sounds like a fan or motor is running. It is a buzzing or humming kind of sound. This noise is loud enough that I can hear it in the car at idle and can hear it 15 feet away from the parked car. This noise is occasional and I have no idea how to turn it off or what it is. When I have this noise it is persistent and will keep going until the car is parked 10 minutes or so, then it shuts off. I may not hear it again for weeks. It comes from the very front driver's corner area, just behind the bumper. Please solve this mystery, I cannot find the answer online anywhere.

It's the electric fan. It continues to run to cool down the engine. It keeps the engine from getting hotter after you shut it off.

It's not the fan. I had the noise come on again the other day. It is a higher pitched noise than the cooling fan. It is from the front driver's corner of the engine compartment, just in front of the airbox or below it. Any other guesses? It sounds bad from a distance and I can even hear it when driving if I turn down the sound system.

I believe there is some type of fan that runs to minimize the condensation in the a/c compressor.

That is the heater fan. Will come on 50mins after being parked, and run for 5 minutes.

Thank you for these possibilities. By heater fan, are you referring to the climate system AC/heat fan that blows air to the vents? I have heard the cabin air blower fan come on when the car is off and it is not loud like this sporadic noise. This noise is annoyingly loud. Like a humming buzzing from a motor. My best guess would be that it is some system that processes fuel vapors or exhaust gas. It seems to be a 1-in-a million noise because I cannot find anything helpful by Googling the problem.

Try getting piece of tubing a few feet long and put one end up to your ear and move the other end around the area you think it's coming from. Find the culprit, snap a photo and post it here. Best way I can think of finding out what it is. All I can think of that would buzz in that area would be a bad HID ballast, but you said it's even while the car is off...

I tracked down the noise. I have no idea what this component is. Just forward of the airbox, beneath a few wire relays is this silver dome. This is where the humming comes from. I can see it is shiny silver, about 2.5" diameter. Says something like Volvo Pumps U928 or U958. On top is a sticker that says T3. What is this thing and how do I remedy it being so noisy?

Okay, making progress. This noise maker appears to be the vacuum pump. Info is still hard to find on how to remedy this problem. It is a fairly common question with few answers, or links to non-English websites.

Maybe cruise motor?

Yeah, I think this air pump provides vacuum for the cruise & brakes. Is anything wrong with it and why does it sometimes run so loudly with car on and off? I can not find these answers online. I also don't know why the email notifications are not reaching me from this forum. I will check back periodically. Thanks for your efforts to assist. Silver S80

There is a valve in the hard plastic line that goes bad and causes the pump to run more than it should.

It is my vacuum pump. The problem seems to be worse in the winter, it is fine now. It makes the sound you hear when pressing the brake pedal, but would just keep making that noise for 10 minutes after the car was off. I will leave it be until the problem resurfaces.

Most likely the electrical valve in the one hose that leads to the pump is bad.