I am not intimately familiar with this engine, but in general if it makes more noise on the overrun, then it means that it is the conrod bearings. Also the sound it makes sounds a lot like loose conrod bearings. They are not that hard to replace even with the engine in the car, as long as you can take the oil pan off.
I was able to take the oil pan off my w108 (m117 engine) by loosening the engine mounts and jacking up the engine.

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Csaba
1972 280SEL 4.5, silver