I put this 1996 351W back together from a block, and have had this problem with the valve train since putting it back together. NOISE. Clackety Clackety Clack. I put in new lifter rods, hydraulic roller lifters, new rockers, new valves, the whole 9. Noisy as heck.
I'm on a hunt to find out if they're getting enough oil. I have the valve covers off and am turning the engine by the starter motor, and no oil is coming out of the push rods, but the valves themselves (after running the engine a few 1000 miles) do have oil in their cups. Not a lot, but some. When I put the engine together, I primed the oil pump with my drill and back THEN I saw oil come out of the lifter rods. But it's not coming out right now. Not at engine crank speed.
... should it be coming out?
The lifters all appear "pumped up", there is literally no space between any of the lifters and the valve stems. I don't know how long it takes for them to go back down, but currently they're all lifted.