Loose Exhaust hangers, could cause an RPM specific resonance, Flex Discs would cause a road-speed specific resonance. Since the fan clutch is new, it can be ruled out to the point of maybe there is a pulley that is cocked slightly off kilter causing it to resonate.
Does the sound change as the engine warms up at all?

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2002 E430 4matic (212,000km)
2002 ML500 'sport'

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