Timmy2, if you rarely start the engine in this 1988 420SEL the timing chain tensioner likely loses oil pressure which results in a slightly slack timing chain at start-up. That would cause the brief rattle you hear. A new timing chain tensioner, or a rebuild (if that's even possible) should resolve the problem. I am unfamiliar with MB 8-cylinder engines, but I doubt that removing the valve covers will expose the timing chain. As far as I know, you would need to remove the upper timing chain cover to view the timing chain and upper guides.

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Fred Hoelzle