How these engines develop noises that dissapear is almost beyond me. If you have a noisy rod it should be louder after the oil is really thinned out. One day I even had a suspect cracked flex plate. When that sound self corrected a couple of days later I kind of threw in the towel. If your rod theory concerns you loosen off each injector in turn. It should locate the knock. Then change the injector with the one adjacent to it and see if the knock still exists on that cylinder when you loosen the injector line of the cylinder or has it moved. The main thing with these engines is not to get too excited I guess. I would pay a little more attention if it were number one cylinder though. If it is a rod problem for some reason it is usually number one. Not written in stone though.