I've speculated that if it doesn't quiet down when running on VO, then points more to lifters. Otherwise, if it goes away on VO, points to injectors and possibly IP timing.

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1982 300TD 210K miles ("The Replacement" aka "The Anvil") - SOLD
1979 300SD 245K miles (never ending project)
2007 Pinarello F3:13
1995 Ducati 916 (SOLD, sniff)
1999 Ducati 900SSie (SOLD)