Consider pullin those valve covers and taking a look at the springs. If they're stock springs, they can break after sitting for long periods of time, especially if they've been mishandled and damaged or dropped. While you're in there, find TDC compression stroke for #2 and shake the rocker assembly. It should have a little slack like the others. Anything looser than the rest and the cam and lifter is no good. Regular ATF is a good oil detergent. I've never heard a noisy lifter last longer than 20 seconds on an engine with good oil pressure. It's likely a flat lobe. The pushrods can also punch through the stock rockers. If it's ever been over revved. This can all cause noise. Don't continue to run it until it's corrected, or it'll flatten the cam for sure.