I too, hear the same noise when I get going first thing in the morning with my 93 300E. I probably should have investigated it since it has done so since I bought the car at 72K miles.
Good to know it's a normal function...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car