Ditto on the resonance theory - I have a terrible rattle I hear (seems to rattle at about 10 Hz) that I only hear at 950 +/- rpm. I finally figured out that it is one of my air cleaner mounts - the nut is missing on one of the corners and the rattle of the bolt in the mounting hole is surprisingly loud.

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1984 300D Turbo, 326000 Km, Thistle Green Metallic