Strange Exhaust Noise - Help with diagnosis please

[Gambit]

That first spark plug doesn't look like it is firing. It looks very clean.

Second plug looks like a slight lean condition. A touch of white ash forming on the tip.

Third plug looks like it may not be firing all the time, gas is washing carbon off the plug.

Fourth plug is what a spark plug should look like, slight brown color with a slight bit of carbon.

Now we know what they look like at idle, when you have an opportunity, take it for a short drive. Get it up to temperature and when you park it, immediately shut the engine. Don't let it idle, that will allow the gas to wash the first and third plug and skew the results.

How many miles on the engine?

The diesel engines are shim adjusted, I believe there is a valve adjustment on the gasoline engines. When you pull the valve cover to replace the seals, it might be a good idea to do a valve adjustment. Might be good to do a compression check as well.

Is this a carbureted or fuel injected engine? If fuel injected, you might check out the injectors. You might have 1 or 3 that aren't quite doing the job (at least at idle). Good candidates for cleaning or replacement.

I'm not denying the possibility of an exhaust issue, but I'm trying to think through other possibilities as well.

Thanks this is all super helpful.

On the plugs - wouldn't pulling each one and having the idle change mean that they were all firing? I'm happy to pull them again and take a look without letting them idle.

These were pulled with engine up to temp (180). Engine has 310k miles But had top and bottom end done recently.

Injectors were just cleaned while intake manifold was off for the Auxillary air valve replacement... so I'm hoping they are OK, but not ruling them out completely yet. I had removed the intake manifold and done the air valve - which didn't fix another issue I was having (cold start & high idle along with this exhaust sound), so I took it to a shop who thought it was all the same issues I had, they figured I screwed something up in there. So they also removed the intake manifold and while they were in there cleaned the injectors (all of this did nothing to remedy the current issues)