Mine does something like this, it seems to make the most noise at 1,200rpm, and sounds fine over 1,800. Mine also vibrates a lot at idle, which my mechanic assured me was just the engine mounts. If you figure it out great, but I don't think it's going to stop the car from going thousands or hundreds of thousands of more miles. Are you getting good power and economy?

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1990 300d 2.5 turbo
1995 E300d