Engine knock

A friend of mine came to me with a very loud knock coming form the top engine around the 3rd cylinder (with engine above idle speed only). All the research I did told me it was a blown connecting rod. I did a full rebuild and found oil starved and slipped brngs. No other major damage. Crank shaft was fine, pistons were not cracked/broken (one piston was a little stiff), cam and bearngs were fine, some lifting rods were dry, but no damage. Cleaned and replaced everything as per a normal rebuild. I got the engine back together and installed in the van, fired right up and wouldn't you know the knock was still there. PLEASE HELP