I removed the aux fan on my 97 e420 using MattLs step-by-step instructions (thanks Matt!). The belt was inverted and found to be the source of the noise. I replaced with a new belt, lubricated where opportunity allowed (minimal) and reinstalled. I took the opportunity to also clean the condensor and surrounding areas. Fan works like new - nice and quiet. I wonder how the belt finds intself "flipping" over time?

Thanks for all who offered me a reply. Each and everyone was a help.

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Woody