...go to member "JimF' website. He has listed a number of various issues dealing with MB cars. If you look under issue #18 he describes a situation similar to what you describe. He also lists a solution to the problem.
My W126 420SEL makes a similar sound. It happens mostly when it is cold and it doesn't happen all the time.
Hope this helps you.
PS: By the way, if your car is running hot, check out Jim's "cool harness." My daughter's 300TE was running hotter than I liked. I lower the coolant to water ratio to 45/55, added WaterWetter and installed a cool harness. Her 300TE has been running cool and smooth, even in the recent 100+ degree heat wave. check it out.
1989 300TE "Alice"
1990 300CE "Sam Spade"
1991 300CE "Beowulf" RIP (06.1991 - 10.10.2007)
1998 E320 "Orson"
2002 C320 Wagon "Molly Fox"
Res non semper sunt quae esse videntur
Not in this weather!