If you can hear the air-pump spinning, I would definitely check all the vaccuum lines and their rubber connectors.

I was getting this same symptom and code on our 1993 500E (same engine & emissions) and when I removed the vaccuum line to the air-pump (hard to reach and see) and the rubber elbow connector had a tear in it.

I purchased a bunch of them (cheap) and replaced all the rubber connectors I could find due to the extreme heat and age these pieces are under.

No more CHECK ENGINE.

:-) neil
1988 E36T AMG
1993 500E