I had a '91 W124 Euro 300D 6cyl no turbo. I was sure I had a tappet problem. Replaced all the tappets and cam followers. Rattle still there. It turned out to be a part on the little shock absorber on the front of the engine. A plastic sleeve had deteriorated and I had metal on metal. Machined a replica on the spot with stainless. Problem solved!

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'97 E300Turbodiesel "Superchip" 200+bhp
'00 A140 Avantgarde Auto
'91 500SEL W126
'66 220SEb Coupe (restoration pending)
'58 180D (restoration underway)