[jhv48]

Similar symptoms when I put a custom ground cam in an engine. Every few miles, the clatter would show up. Adjust the valves, clatter gone. Came right back after driving again. Adjust, repeat. Checked push rods and lifters several times. Always ok.
Turned out the cam was "going flat" the lobes weren't properly hardened and weren't holding their shape. So, as they wore down, the noise returned.
Replaced the cam, all was fine.