Fan Clutch Problem

I have a 1995 BMW 740iL, a couple of days ago as I drove on the interstate, I heard a loud humming sound that only got worst as I accelerated, I thought it was the transmission, yet the gears engage
correctly. So I stopped at a mechanics and he came to the conclusion that it was the fan clutch that it was locked. The question is will a bad fan clutch make a loud humming sound, it sounds as if the engine were being stressed out, yet it drives fine and the transmission is engaging correctly. Can anyone shed some light?
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The fan clutch has nothing to do with the transmission - it's a viscous coupling (clutch) to the radiator cooling fan which should progressively lock up if needed on hot days, etc, thus only spinning the fan fast when needed. This saves both energy and noise. And they can fail in the locked state. It's easy to check. With the engine cold and not running, try spinning the fan by hand. When working correctly it will spin freely.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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