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M50 Knock/Thump

Nice subject, eh?

My '93 E34 525iT is making a knock/thumping noise.  The frequency of this 
noise directly corresponds with the rotation of the serpentine belt.  So, at 
idle, for each revolution of the belt, I'll hear this noise.  As engine RPMs 
increase, the frequency of the noise increases.

At idle (~700 RPM), the frequency of the noise is about four/second, which 
would translate to 240/minute, or a rate of about 1/3 of engine RPM.

I don't know what would be running at this rate.  Is it possible that a belt 
driven accessory (power steering pump, alternator, water pump) is making 
this noise?  How about the belt tensioner?

Also, I don't know if it's related, but the fan clutch is shot.  I can 
easily stop the fan from spinning if I stick a rolled up newspaper into the 
fan blades.

Thanks in advance for your info/advice.

- -rb


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