Diesel knock

Is diesel Knock harmful to the engine?

Do you mean at idle and light running, or when "tuning"?

The former, no - assuming it is actually the fuelling and not an actual mechanical issue, the latter is usually as a result of over-advanced injector timing, and is like over-advanced ignition, which is a bad thing.

Just following up on Gords great reply, this webinar is worth a watch for more information and some demonstration on the dyno: https://www.hpacademy.com/previous-webinars/161-understanding-pilot-pulse-diesel-tuning/

This also touches on it in passing: https://www.hpacademy.com/previous-webinars/159-diesel-tuning-fundamentals/