I have a 93 400sel. When A/c is on or heat for that matter and the car is at idle, everything seems to work fine. Heat or A/C blows out all vents. When I accelerate, air from vents decreases considerably and there is a hissing sound. A buddy of mine said it is a vaccum leak. I can hear the pump under the back seat trying to do something. Is this something I can try to trace or do I need to take it to a shop for repairs.
Thanks for any suggestions.

 

This is not a job for the novice. Sorry but you need to visit the shop most likely. One thing though. Pop the hood and check on the passenger side near the windshield, there should be a vacuum line (green I think) pretty obvious, not hidden really. Make sure this is connected and not broken.