I had the vacuum pump making buzzing sounds till it got me very mad. It would go on as soon as I start my 1994 C180. The noise is from the back boot under the fuel entry pipe and thats where the pump sits. The only problem was the rear headrests won't drop when I press the button on the dash. After a very expensive pulling off of the dash, they found 2 plastic socket things with rubber fabric inside which were cracked and holed. These changed, the pump is quiet now, saved $1000 for replacement, rear headrests work now and fall quickly. However, my aircon now acts strange. When I accelerate, the compressor goes off, get hot air or no cold air. With the accelerator off or released, it comes on again. For one year the aircon worked correctly after the vacuum fix.