I had the same noise and finally pinpointed it to the belt tensioner. An easy way to identify the culprit is as follows: While the engine is running, using a broom stick, VERY CAREFULLY, push the broom stick hard against the side of the belt tensioner. If the noise stops, you identified the problem. Repeat this process with the Belt Tensioner shock absorber.

Phil

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