Oil filter. In a pinch I've used NAPA Gold (Wix) filters, and they do this on my 300E, about 2 seconds of tappet noise on cold startup. When I use a Mann/Bosch filter, no such problems.

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http://i193.photobucket.com/albums/z...-RESIZED-1.jpg
1991 300E - 212K and rising fast...