pipe popping

Popping in the exhaust is the introduction of oxygen in the pipe where the exhaust is still hot enough to cause a secondary combustion. If it is a stuck valve and it calms down when heated up (this is not how sticking usually works) and doesn't get any worse when you adjust it you should send that valve to a metallurgist, because it's got something special goin' on in there.

So, headers or manifolds?

Left side is the side that hangs low on cheap headers, and provides a safe "dry docking" action against large bumps. Left side is also the hardest side to tighten the rear bolts. If you have power brakes, the whole area is hard to even look at. If you have power steering, that's the side where the steering box smacks the headers around (or where somebody had to smack em' with a hammer to make them fit). It is literally the perfect place to get an exhaust leak. If it's small enough that it doesn't happen at idle and it goes away after heating up, it's probably not big enough to actually feel or hear a puff out of, but as soon as you get some gas velocity over the crack, it does a little venturi effect and sucks in some air. Once the engine is up to temp, the gasket is heated up and the crack shuts.

I bet it's the exhaust gasket, because it's always the exhaust gasket. And it's always the exhaust gasket between 5/7 right where there's no good way to get in there.