GLKwanter

GLK 350 in a warm, quiet garage. 101F out today. Got home over an hour ago, and I just noticed a very quiet fan noise coming from the passenger side firewall area. Wife said, how'd you even hear that. Ear to the black plastic between the windshield and back hood edge hears it best. Open the door, the cabin light comes on, and the fan turns off. Close door, it turns back on. I don't know, but seems too quit, small, and out of place to be engine coolant related, which is all my searches find about fans staying on after the car is off.
It finally STAYED off after I got in the passenger side, with no dash lights just cabin light, and pushed the off button on the air controller then got out and shut the door. Didn't turn back on. Maybe coincidence. I think I heard the CD player cycle too when I got in, and I never use it. I had no key nearby.

Fans off now, but what fan was it? No air was blowing through the vents. Maybe it's not a fan, but what would run that long?

Mmr1

Maybe the electric water pump, or another pump, or the blower, hopefully it’s just the excessive heat , I would monitor it and get a volt meter , battery tester to be sure the battery can handle it, is any air moving from any vents? Whatever it is could drain your battery . Do you have a Charger? Get a cheap maintainer charger that works on AGM battery, Noco and CTEK are good. You can leave them on forever to maintain the charge. A volt meter or cheap load tester will tell you battery condition. There are several good load testers for 50$ or so. Only a load tester will tell of a battery’s overall condition. If it continues when cooler or if your battery is suffering have a pro look at it. You don’t want to get stranded. 4 good things to have a jump box, a good charger like CTEK, and a load tester. And tire pump. 300$ can get you set up.

GLKwanter

No air was coming out the vents. Car wash off. I took a video at the time, can barely hear it. This is passenger side, above the firewall, between windshield and hood. If it was a blower it sure is quiet. I still don't know what it is right there in that vicinity
https://youtube.com/shorts/rVzAwfzaiGI?feature=share3

John CC

Probably a cooling fan for some electronics. I think I've heard the same thing.

Mmr1

Remove the battery cover if it’s at the windshield passenger side , on top, you will see the vent system , the ac blower is inside there and you will hear it better if that’s the problem. My coolant water pump or something also runs after the motor is turned off, but not for an hour. Your local auto parts store will usually run codes for free , (to sell you the parts) go to a store that has no service center, they will do it quickly.

GLK-K

Newbie here. The exact same sound and location happened with my 250 GLK BlueTec a few days ago. I placed hand over vents and felt very very slight air movement. I pressed off button to air system and it stopped. While car was off. All I can think of is 1) temps in Houston have been 100 for days or 2) left air in ON position when ignition turned off.