if the rockers (both banks) are being fed then the horizontle main galleries & therefore the lifters (which are upstream) are being fed also. I'm assuming you did a cam breakin procedure when you dropped the new lifters (unrelated to the noise tho). Only thing coming to mind is, see what your preload is: get a lifter on base circle. Set a small thin 6" steel scale ruler across on the valve cover rails close to the pushrod. make a sharpie mark on the ruler edge & slide it over fast to make a witness mark on the pushrod. unbolt the rocker shaft evenly so it rises evenly (horizontally) till the pushrod JUST stops moving upward )if it goes sideways it'll skew your test). repeat a witness mark then remove the rocker shaft/rockers all the way off & lift out the pushrod & measure the distance between the two marks. you're sure nothing else is rubbing/touching correct & that it is lifter clatter? I wouldn't think the preload would be borderline not enough then something finally reduced it that last bit/locked it solid (in effect no preload). Holler how it turns out