It sounds like the check valve in at least one of the lifters is failing to seat, allowing the lifter to partially or fully bleed down. Try adding a solvent-type oil additive a couple of hundred miles before doing an oil change. Rislone, Casite, or Wynn's Friction Proofing were brands I used 30-something years ago, but I haven't looked for anything like that on the shelf for a long time. Early Stovebolt hydraulic lifters were notorious for being noisy at times- - - -most of us just accepted it and didn't get too concerned when it happened. Sometimes a change to a different brand or weight of oil will solve the problem. If he's using a water-thin lube like 10W-30 or thinner, the lifters will definitely be noisy.
Jerry
