2012 Mercedes-Benz E-Class E350 Sport 4MATIC 4dr Sedan AWD (3.5L 6cyl 7A)
best guess is intermittent defective oil pump .
on half the start ups ( cold or hot ) , loud valve taps / rattle ... most likely due to oil lying in the oil pan . seems an odd explanation , as benz uses synthetic oil that sticks to moving parts ?
Benz gasoline engines , as long as i have owned them ( 45 years ) , have quirks .
Mitchell, I have extactly the same problem. I just had the car in for service where I asked them to check it out, and their response was that these cars just have very loud engines. The high compression of the Blue Efficiency engines makes them comparable to diesel engines, and as such they are loud. While I am not sure that I buy that answer, I don't feel that valve tap or chatter is normal in any engine. Have you found out anything further?