I also have a knock that sounds like yours. It happens at every rpm and load and came without warning. I just pulled into a rest stop to get a drink and i noticed an odd sound. I have replaced such things as the injectors, precombustion chamber, delivery valve (Big performance boost ), and set cam/IP timing.

To date, only the delivery valve has made ANY difference in the knock and performance. I have not checked the main/crank bearings yet, I'll be doing that on my next oil change in March.

Have you "cracked open" an injection line to see if it goes away? Swap injectors with a cylinder across from the problem cylinder (I.E. #1 swap wth #5, #4 swap with #1, etc.). If it follows the injector, just replace it. If it dosen't, you're in for some "fun."