I had a similar issue with my 560SEL, which was leaking oil out of the valve cover gasket, and one of the lifters would tap on start up until warm (and I assume well lubricated). After installing a new gasket and an oil change, the noise went away after a few spirited drives.

My noise went away, and hasn't returned, but I couldn't say the same for your case...

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1987 560SEL (210,000 miles)