Might be as mentioned that the new set of lifters (would only take one or two bad ones to make a racket) are bad. If your preload is good and your psi is good and your oil weight is good, not much left. I did have noisy ones on a later /6 with hyd lifters. it would come and go (mostly at idle) but usually when started & I just lived with it. EDIT & when it would come and go randomly the psi was OK. If I ran into that I'd take apart each lifter/clean/reassemble the pieces (same innards in same body & as you know same lifter back in same hole) & if that did not do it then I'd spring for a nother set & do the cam breakin procedure