yeah, you can rule out the vacuum pump as a candidate, as it vents internally on the SD's it could easily be the catch can in the air cleaner, so be sure and follow the instructions I laid out in the other thread about the catch can to cut down on the oil in the air filter.
http://www.peachparts.com/shopforum/showthread.php?p=2586529#post2586529

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!