You probably have one or more worn Hydraulic Lift Compensators (Lifters). I had to replace a couple noisy ones on my 560SEL. Not difficult at all. Can be done in an hour or so if you have a valve spring compressor tool. It's usually best to replace them all at the same time.

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles