Unless you are a serious home technician, I would follow Steve's advice and have it hauled to a shop. The roller from the vacuum pump could be rattling around, or anything.
If you were to take off the valve cover, MAYBE you would find something obvious. If you don't find something totally obvious, send it to a competent tech that has lots of experience with the breed. Don't take a chance and run it any more without determining exactly what the problem is.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in