The statement that the car was making a rattling sound is crucial to this diagnosis. If it ran for a while when it was rattling, there could be a lot more wrong with the engine than an oil pump. You need to come to a preliminary conclusion about whether or not there is extensive damage before replacing the oil pump if that is what it is.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13