Could be the alternator bearings. Try duplicating the noise under the same condition as before, but put all of the electrical consumables on, such as the heater fan to Max, Rear defroster, Wipers, Lights (on high beam), lighter, etc., See if this makes a difference to the squeal, especially if it now occurs at a different RPM.

If yes than its likely your Alternator bearings, if not, could also be the water-pump.

Phil

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