Pull the valve cover.
Locate a loose rocker on both the intake rocker rail, and exhaust rocker rail. Move them over a little bit against one of the springs.

See if there is wear on either of the long rods under where the rocker resides and does its job.

I had this same issue. I "tightened" the valve lash. Noise came back, tightened again, etc. etc. until one morning there was a really LOUD knocking noise. I'm guessing one or more of the exhaust valves wasn't opening properly anymore.

Anyways, 2 new rocker rods, all new rockers, never a problem since. Worth a shot anyways.