Yes, it was actually defective. The intermittent clicking/knocking sound was present again after the next start, and disappeared when the injector was disconnected.
The injector was removed and tested on a nozzle tester, and it turns out that it's leaking in the return flow direction.
Then I heated the injector with hot air, activated it a few times, and at the return line, more fluid is coming out than at the nozzle

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