Just thought of something... Is it more apt to do it when it's cold and calms down when it's hot? I mean it may not go away completely, but it's not as bad when it's warmed up/hot? I'm thinking it might be piston slap possibly?? How many miles on the block and the pistons? I know you said you just rebuilt it, but that doesn't say how many miles are on those parts... It's still holding oil pressure, so that throws off the bottom end, that's why I'm wondering about piston slap, worn pistons or worn bore that should have been bored out and had replacement pistons put back??