You can always "crack" the injector line loose and see if the sound stops. I've done this on several 617 engines to phish out bad injectors and odd nailing sounds. If anything, it serves as a step in the process of elimination.

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Vehicles:
2002 SLK 230
Gone but not forgotten:
1983 300D
1981 El Camino 'OILBRNR' - 6.2L diesel
OM617 powered '86 F150
1984 BMW 524td
2001 VW Beetle TDI
1994 Sunurban 4x4 6.5L diesel