This engine has a couple of items that can make a knocking noise, other than just injectors. I had a really obnoxious knocking from my serpentine belt tensioner when it died. I also had an alternator go bad and it would do something I still don't understand - I believe it had an internal short or something in the regulator that was shorting and it would periodically act like it was going to seize up, and make the belt tensioner freak out.

I have 162,000 miles on mine and have "eaten" two of those tensioners and one alternator. I presently have some delivery valve seal leakage, but no knocking. And as far as I can tell, I have the original injectors. I know they are the same as when I bought the car with 41,000 miles on it.

Good luck. If it isn't smoking or getting horrible gas mileage or using oil at an alarming rate, I would not suspect anything internal has gone wrong. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)