I just went through this on a 1986 190E 2.3-16. On this car the fuel pump buzzing noise was accompanied by the pump running longer and longer between intervals of being shut off. The fuel pump has an integral check valve assembly, and it is my belief the fuel pump check valve is the buzzing problem. When the check valve fails the pressure in the discharge line is not maintained and the pump runs nearly continually. The motor in the pump is not a continuous duty rated motor and it degrades the longer it runs in this condition. Ours just failed and replacing it resulted in a three second or less zipping noise, followed by a lengthy shut off period. Even when running the check valve noise was very slight. I will dissect the old unit and check the valve guts out this weekend. 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)