I had the exact same symptoms and noise from one of the fans this week. The noise, which initially I thought could be a defective bearing, turned out to be paint peeling off from the inside of the fan shroud.

The fan would then touch the chunks of peeling paint and make the noise that you describe. I peeled off, most of the paint, and both fans now works fine.

Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]