Anything that causes a gasser to detonate makes it sound like a diesel, the way things should sound by the way, but I digress.
Trying running higher octane fuel to see if it diminishes.

Piston slap and worn crank bearings can also make diesel noises. Lotsa clacking in the engine.

Valve issues aren't really "diesel" sounds, because its the same set of issues for gassers. These are more top end clickety sounds.

My diesel was developing a clickety / ticking sound, using my stethoscope zeroed in on cyl#2, top end. The sound went away after a thousand miles on a new brand of oil and filter change. Shell Rotella 15w40 if you must know. I had shop grade 15w40 diesel oil in previously, and I think my Mercedes was just telling me to stop being cheap. So I bucked up the extra 25% money on good stuff... Ticky went away.

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Cheers!
Scott McPhee

1987 300D