Getting a popping sound out of the passenger side exhaust. Number 8 ex. rocker was loose. Bent valve is my guess. Anything else it could be?

Broken valve spring will do that too...

It could be fuel burning in the pipe and the cold air rushing back in.

If its popping when you let off at the big end, it could be a bad header gasket.

The popping was at idle. When i reved it up it sounded like it was gone.No popping when i left off on the top end.

When I had a same issue it was too fat, fuel re igniting in the
exhaust

camshaft going flat if its not a roller

i had the same issue with the 408. when i pulled the heads all exhaust valves had been hitting the pistons. the springs got weak and they had over .200" to piston clearance. i'm thinking you may have an exhaust valve or valves not seating.

The valve springs are probably week. Seat checked 210 not sure open pressure. Its a roller. Springs have been on the motor since 2008. 700+ runs. Maybe more.

I had a similar issue, popping out the exhaust only at idle. Bring up the RPM's and it went away. I checked the ignition pulled carb apart, adjusted valves, checked for intake and exhaust leaks nothing worked. In the end..bad gas. Emptied the tank, cleaned it and put in new fuel and it ran like a champ. Gas of today isn't anywhere near as stable as fuel even from a few years back. Something else for you to consider.

I'm going to get the motor out this week but im starting to think it a bad roller lifter.