I have searched for an answer to this and have not found what I thought this could be:

1996 C280... after startup, hot or cold, if the climate control is on, in any position, any temp, and you switch it off... you hear the urrr-urrr-urrr of the vacuum pump in the trunk cycling... I can get it to do this just about any time... a little frustrating...

Central locks work fine, back headrest drop fine, center air vents blow fine, most of the time... sometimes, riding down the road, the center vents suddenly put out more air than they were... meaning, if the fan is on medium, there is air coming out of the center vents, but after a while, you suddenly notice AIR coming out of the vents, almost as if the fan was turned up...

Something else that I think is tied in to noise... after driving a while, if sitting at a stop (probably does it at all times, just cannot hear it over road noise) you can hear a tap - tap - tap from under the dash, like a flap closes and then closes again and then closes again... on some occasions, changing the temp, you have to crank it all the way hot before it mixes much warmth at all, then can back down on the heat and it mixes correctly... but to get it from cold to warm up, you have to go hot, then come down...

Any help or advice or guidance will be greatly appreciated!!!

Thank you!!

 

My car did the same thing when one of my vacuum elements under the dash had a small leak in it. I could hear a psssssss sound and then a second later the vacuum pump would kick in for a couple seconds.. then it would repeat. Ended up pullin the dashboard out to get to the sucker, replaced the vacuum servo and it works like a champ now.

 

common problem, you have a vacuum leak behind your dash, one or more vavuum elements are leaking, fix it before your central locking pump burns out.