My erstwhile '84 190E 2.3 had an electromagnetic clutch on the engine driven fan rather than the thermoclutch as is the case with my 2.6. It sounds like they stuck with the electromagmetic clutch on the 102 four-cylinder engine in later years.
I feel you might trying to fix a "problem" that doesn't exist. Assuming you live in Europe you had an unusually warm summer and the fan was probably engaged a lot.
On my 2.6 when the temp reaches 100C, which is not often in my driving conditions, I can definitely hear fan roar at and above about 2000 RPM when the thermoclutch tightens. When the car gets hot more air flow is required through the radiator, and the only way to achieve this at low speed is with fans, and fans make noise. It's the nature of the beast.
The only thing you need to check is the electromaganetic clutch. I expect that if they fail they would fail open and the fan will not engage though it's possible that they fail closed. If the system is designed such that energizing the clutch disengaged the fan this would be a better failsafe design as failure of the clutch or control system would keep the fan engaged all the time and prevent the car from overheating. The fan should spin freely when the engine is cold, and this should be checked both with the engine off and on, but cold. If the fan is engaged with the engine cold then you will need to troubleshoot the clutch and control system.
Assuming the system is operating normally as we progress into fall and winter, your fan roar issue will go away with the cooler temperatures.