The good news: Replacing hydraulic lifters is easy and cheap ($10). I had one collapse and when I watched my mechanic replace it I thought, "I could have done this in 10 minutes."

To identify the faulty one(s) you need to press down on all of them in the rocker arms(?) until you find the one that is loose. Unbolt that arm, pop out the old lifter, insert the new one, and replace the arm. That's it.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black