I did the overhaul of my OM603.971 one month ago.It had that sound,and it turned out that it is a broken piston slamming on the cylinder wall .
Hope that this is not the case with yours,but it sure sounds the same .
And I had no change in the oil consumption,fuel consumption,engine power,no smoke from the exhaust,nothing except the loud knocking.

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Mercedes W140, S350TD, 95.
Mercedes W140, S350, 92.
Porsche 944, 86 N/A.
Porsche 944, 85 N/A automatic.
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Opel GT, 73.
Toyota Supra MA61,84.