Grinding, " diesel " noise under valve covers

I maintain you still have rocker issues .
If you try to pinch two adjacent rocker together while it's running, I think you'll find they try to spread apart, and "clack" back together.
I also suspect that if you take the rocker/shafts off for inspection, they'll be interference marks on the side of the pshrods and rockers .
Won't take much to check, and replace header gskts while you're at it .
Lotsa posts .
Good luck !