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It is definitely possible that an injector can cause a knock noise. If an injector has failed internally then it can cause excessive fuel to be injected and it will cause a fuel knock. The should be able to do an injector kill test which allows then to kill each fuel injector one at a time with it running. If the noise goes away with a particular injector killed then that injector has likely failed. It's possible that you could have a rod bearing or piston issue and the noise would usually get better with the injector killed, but if it goes away completely then replacing the injector should take care of the noise.