<VV> noisy lifter

[Michael Kovacs]

 Tearing my hair out and I don't have too much left.      A friends Corvair. 
 Problem: an otherwise smooth running 102 PG has a loud engine "clack".
 History: rebuilt 102 engine several years ago with no $ spared. All parts from Clark. Lifters, cam, rocker arms, push rods, springs etc. Very low mileage after the restoration.
 The engine seems to have developed one noisy lifter. It is loud like the rocker arm wasn't tight enough, leaving clearance between the rocker arm and the valve.
We first did a hot adjustment, as it was convenient to do that. That did not seem to work. Same noise
 Today we did a very careful cold adjustment by the book using 3/4 turn down after lash was removed, changed oil and filter. Same old noise. The only thing I could see was that one nut on a rocker did not seem to be turned down the usual 3-4 threads after adjustment (maybe 2.5).
 I've rebuilt and adjusted many valve trains and now I'm stumped. Would it hurt to slowly turn the one I suspect, down 1 or 1.5 turns hot?
 As an aggressive alternate, do you think putting different lifter in the one I suspect prove anything if all else fails?