The noise sounds like hydraulic lifters not filled with oil. After our 1998 E320 sits for an extended period the lifters make a slight ticking sound when first starting the engine with 61,000 miles and Mobil 1 0W40 oil. If your engine has significantly more miles the lifters may make more noise. I can't say from experience. Seriously doubt the pistons are hitting the valves ... that process would quickly destroy the engine as far as I know.

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Fred Hoelzle