That knock sounds to me like top-end, so could be bad injector, or maybe a bad valve-train component. I know a rod knock is a deep sound, and typically shortly after you hear it, the rod will exit the block.

As suggested, try disabling injectors one at a time to isolate the cylinder. Move the indicated injector to a new hole to see if the noise follows. Carefully inspect the valve followers etc. for that cylinder.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC