Could be a spark/arcing noise, an exhaust leak, belt noise or valvetrain. Try to listen to locate the noise by holding a rubber hose to points on the engine and to your ear. Don't let the far end hit the rotating fan! It will be a deafening racket...been there.

We're the lifters replaced, if not did they make noise before pulling the cylinder head?

When the rockers were torqued in place was each respective cam lobe on its base circle vs. high spot? These engines sometimes pull out the rocker arm assembly threads into head. There was a modification to slightly longer bolts to prevent this. A rocker assembly may have pulled loose.

Last step would be to pull the valve cover and feel for a rocker arm with excess valve clearance when the cam is on that rockers base circle indicating either a problem with the rocker arm bolts or the lifter.

Was every small disc/puck on top of the valve definitely reinstalled?

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