I am looking at an '85 300D with 220K on the Odometer. The car starts, runs, shifts and handles well. I checked it over and it needs a couple of glow plugs and the windows don't operate properly..none of which bothers me too much.
However, there is a loud knocking noise coming out of the vacuum pump. It is RPM dependent, and is quieter at some times, but never goes away. It could easily be confused with a rod knock or REALLY loud combustion noise on one cylinder...but the stethescope tells otherwise. I am an experienced mechanic, with absolutely no Mercedes experience. Is there a common problem with these pumps? Are they rebuildable? Can they damage anything in the engine if they fail?

Thanks for any replies.
Joe