Side play in rockers will make an awful racket. When you are adjusting your valves try to wiggle the rockers sideways on the rocker shaft while still at top dead center, and set at .006. They should just move a wee bit. If you have one or two that you can slap around when loose, that's a lotta noise. Read up on shimming them. The rocker pad on the valve should be contacting the valve stem a little bit off center and there is a clearance tolerance so oil can get in. If it's a purportedly fresh rebuild that is what I'd look at, after being absolutely sure you have the valves adjusted properly
They will make noise for a while after you adjust them, so you may want to check on them after a while. Noise should go away with in 3 days max, depending on how often you drive. If I was you, i'd go to hydraulic though. In which case you wont set them the same way at all, lil harder but worth it in the long run in my opinion.