Are you referring to the electric fan in front of the condenser or the belt driven fan on the water pump?

Electric fan operates off coolant high temperature or high a/c system pressure.

Mechanical fan has a viscous clutch that normally engages on high air temperature but on startup the fan will typically operate briefly (due to the oil pooling in the unit at rest?).

Does it roar like a garbage truck? If so that is normal and means you have a properly functioning fan clutch.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech