What all was done exactly? Did you replace the lifters? And if not, how long was the engine down/apart for? I suspect it's just the lifters being full of air. When I went to start my engine after the rebuild, it was knocking/tapping very loudly but slowly began to quite down and eventually went away as the lifters bled themselves and filled completely with oil.
Air in the liftes does little to dampen the noise, it's the oil inside that acts like a shock absorber and keeps them quite.