I don't get what your problem is....your fan clutch is engaged? As long as it's not always engaged and your car isn't overheating leave it alone. I'm sure you have bigger fish to fry, if not then just enjoy the car. I have a small oil leak, it's annoying, but is not worth tearing into the lower engine to replace the gasket. If the fan isn't causing you any detriment while driving the car then you should leave it be. Now if it starts overheating, that is a different story, but because your car is operating as it should with normal temps, save for the fan noise, I'd say you're ok.

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1990 190E 3.0L