I had a similar problem, however I could not hear the pump. My compressor was out and it cost about 800.00 installed with tax, at a private mercedes specialist in Seattle. (Kirkland, Actually) that said, if your compressor is working, but no rise, then I would check for leaks. i am told if you have a leak the pump will not keep running continually, it will shut itself down permanently until reset.
From my experience I would consider a leaky air bladder/strut on one of the wheels, or a leaky hose, which is less likely. That is under a grand to fix if i remember and ebay had some for far less, especially if you are at all a do-it-yourselfer, (I think 450 per each)
be super careful driving the car when it is slammed, especially turning because it is likely riding on the frame bumpers and you risk tearing out a wheel well with a bump. (the clearance was less than my finger.
-Doug 2001 s500