When I replaced the lifters in my 4.8L recently, i ran into a problem with a shop not grinding the valve stem tips when they did the valve job.(this is a whole other story that is really bad in its own right) Anyway, most of the lifters clattered like crazy when i got the heads put back on and the engine fired up. I ended up getting a set of .025'' shorter pushrods to solve my problem. After that, the lifters were nice and quiet. So I would check pushrod length, coil bind, and run it with the valve covers off to see if the noise is maybe just the rockers hitting the valve covers. By the way, if you didn't wash the rockers thoroughly with solvent when you got them, they would most likely have Al shavings on them from machining and assembly.