that's interesting, i did find one loose transmission bolt, which at its rear had a little light bracket holding the speedo cable. i guess this was a dumb hack, because there was a broken plastic hanger nearby where maybe it used to be held. that's an easy fix, but i can't believe it was the whole problem here. FWIW the horrible noise varied with engine speed. i'm still thinking piston ring but no proof yet.

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'77 240D, 504H, OM617.952, etc.