Hmm, knocks are bad mmkay?

The carbon knock is duller than a mechanical knock. When I was in the workshop we had a C36 come in with a knock in it and carbon was the cause. I can distinctly remember the noise it made.
Lifter knock is more like a light rattle and usually goes away as the revs rise because the oil pressure comes up. It won't stop if the cam is worn though but you can very easily inspect the cams by removing the rocker covers.

Then the big one.
I had a knocking in my M103 (6 cylinder) and it sounded exactly like lifters.
However, it turned out to be a spun big end bearing!! You can diagnose piston related know by pulling off the plug leads one by one.

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Andrew
300e 294,000mi
380sel 185,998mi
380se 309,980mi