Removing the cover, given what has already been done, would sound like a good move.
I'd check for looseness in a rocker arm, and if you find a loose one, look around that arm carefully to see if maybe one of the cup-shaped "buttons" that go directly on top of the valve stem maybe flipped out of place and is laying around on top of the head. Seen this once before. Could also be a collapsed hydraulic compensator.

Gilly