I believe the cars with the serpentine belt, and its tensioner, are susceptible to this noise when the tensioner wears out. Check the threads on engine mount replacements of late, there is a link between the condition of the mounts and the life of the critical tensioner parts described that was news to me.

I would guess you have pointed to the right component, but you should make sure you look to see if the mounts are shot too, otherwise it looks like it is possible for the tensioner to have an unsatisfactorily short life. Good luck, 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)