Hey everyone, need some expert help. My series 2a has started making a terrible knocking noise.

Here is a video (not mine) of the same noise and same conditions.
https://youtube.com/clip/Ugkx1a68wuD...S4_KfEwMVnQ0-8

He found the head gasket to be the issue but that doesn't appear to be my issue. So far I have replaced the head gasket, cleaned the cylinder head and piston faces, reset clearances on all tappets, new spark plugs and checked gaps.

The truck starts and idles fine/smoothly. Noise only happens when engine is under load. Noise is the same regardless of engine temp. Feels/sounds like it is in the back end of the engine.

Would appreciate any insight anyone has.