In my experience, Mercedes get noisy rattliing sounds when they need maintenance. Usually not too expensive if you do the repair yourself. Oftentimes, it's the air intake system rattling around.

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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)