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)

Flint's mystery oil added to the engine oil has resolved much valve ticking in my day.

Red bottle

1/2 to one half quart in the oil fill.

So, it is OK to drive with that valve ticking noise, and you can make it go away by

1. Changing the plastic camshaft oil tubes, and
2. Going to synthetic oil.

Is that right ?