I had a loose motor mount (bolt was cross threaded, wouldn't go in all the way and the head stripped, so I said screw it) on my passenger side. Made a horrible knocking noise when Id hit bumps, start the car, and shut it off. Finaly fixed that..

could it be a ball joint? Maybe the rocking of the engine on startup causes it to make the noise also

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1981 300SD 512k OM603