Could be something as simple as carbon buildup on the injector tips. Before digging into the hard parts run a heavy dose of diesel fuel injection cleaner like Power Service, Howe's, Lucas, etc and run the dog piss out of the truck. Don't baby it. See if the knock goes away.
If it still knocks then it might need have the injectors removed, checked and cleaned. Also at the same time check the valve lash.
MikeyB