Well, I'm just guessing here...
Check your belts and pullys. Make sure your induction system is clean and tight.
Listen up front with the engine running. You may be able to zero in on the source.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)