J19 loud knock

New to the forum, I’m rebuilding a series 3 with a J19. The engine has had new pistons, big and little ends, crank etc but still has a really loud metallic knock like someone hitting it with a hammer. The knocking is about 1/4 of the engine rpm and speeds up with the revs. Can’t for the life of me find what it is. Sounds like somewhere in the middle of the block but hard to tell. If anyone has any idea it would be much appreciated. I’m thinking maybe the oil pump? Thanks.

Piston hitting a valve?, Pushrod out of place? Any valve is once every 2 rotations

injector sticking? is it smoking too

Agree maybe an injector , they can knock loudly if the spray pattern not correct, you can slacken the union nuts one by one while engine running to see if noise stops , wear ppe
When you were rebuilding did you turn over engine by hand at intervals as crank was built up to check for free turning
Nice looking rebuild BTW

Head skimmed? hot spots skimmed to match? hot spots tight? correct pistons? new piston protrusion checked against old ones? pump timing over advanced?