I just solved the same problem in my '94 e420 that I have had for a few months. Car would start fine but then develop a ticking noise after fully warmed. Changed the plastic camshaft oil tubes located under the valve covers and, as car was due for an oil change, switched to a 5w-40 fully synthetic oil (I believe that MB recommended a 15w-40 mineral oil and no clue about what car had in it before). Your camshaft lubrication setup may or may not differ because your car has a different engine. Have heard that oil additives with silicone can do more harm than good because they block oil passages. Good luck.

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Brian
'94 E420 - 99K
'12 C300 4 Matic

Previously owned:
'06 CLK 500K Cabriolet
'85 300SD (Old Smokey)
2002 Boxster
2002 C320 (The five-pointed star Mercedes)
1993 300E 3.2 (Dealership's cash cow)
1979 300SD (Chrome beauty)
1983 300D (Old faithful)
1983 240D (Right lane cruiser)
1977 240D (Miracle of bondo)