I have a 1998 E320 wagon with 48,000 miles. It has a mild "knock" from the engine that's most evident when cruising or mild/moderate acceleration. (my wife drives it or I would have found it sooner) It's smooth as a bell at idle and during full acceleration. It's a little quieter during deceleration. Braking or turning have no effect. I added a bottle of Chevron Techron and changed fuel brands, but they had no apparent effect. I had the leveling system and drive-train checked at the dealer and it's fine.
I'm taking it into the dealer next thursday but, what else should I try or look for to help the mechanic diagnose this?
It almost sounds like it's not running on all 6 pistons or only 5 plugs (or would that be 10 or 11 of 12 plugs since it has 2 plugs per cylinder?) or like the dull popping noise of some inorrect valve overlap (but only on one cylinder.
Any suggestions would help>