I own a 1998 E320 since new. The car had tappet noise from the day I bought it when the car was cold. I never addressed the condition with Mercedes since it would stop within 5 seconds after startup. This is indicative of slightly leaky lifters. It now has 62,000 miles and no longer makes any noise. ???? I use synthetic oil and don't tend to change it very often (twice a year)

As a former mechanic I would never recommend engine flush as it tends to get rid of the good desposits as well as the bad. Casuing significantly more issues than you have today. Things may have changed, but from my perspective I would never do this. Hope this helps.