If you suspect the fan, perhaps your engine mounts have collapsed, letting the fan hit the shroud under certain conditions.

Re testing the fan: With the engine warmed up, rev the engine up past 1500-1800 rpm, and as the engine revs up, listen for the fan to couple. You should hear an increase in fan noise.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles