I agree with VSTech. I had 270,000 miles on my om 603 before I replaced the vacuum pump...and I'm glad I did. The bearings looked like they were about to fall apart. There was no discernible difference in noise before or after I changed out the pump. If yours is the old style cover (crimped on without any screws) then you should probably change it just off principle. There really seems to be no way to predict when it might fail.

__________________
97 S500 (90,000 miles) - wife's car
87 300D (298,000 miles and a replacement #14 head)
94 Suburban K2500 - need something to haul firewood
83 300SD (343,000 miles) (sold)