I have a 2001 3500HD with a 6.5. It has a loud knock (sounds like a hammer on concrete). It is most noticable when pressing the accelerator in park and when under acceleration on the road. The sound seemed very much like a piston slap or rod bearing. I replaced the engine with another one. The new engine makes the exact same noise under the exact same conditions on the exact same two cylinders. It is showing a code for #2 misfire. If I crack the fuel line at the injector on #2 or #6 the knock changes a little. If I crack the line on #2 & #6 it completely goes away. When I installed the new engine I used the same computer, turbo, injector pump, fuel lines. I did install new injectors. I also installed new injectors on 2&6 just in case. I understand that these two cylinders receive fuel at the same time from the injector pump. What could possibly cause this? Thanks for any help.