Popping in the exhaust

[tex_waco_usmc]

So, Ive been dealing with this situation and wanted opinions before I go and dump money into the truck.

A little about the truck. It is an 87 TBI 350 with 700R4. I am getting a popping noise in the exhaust. I thought it was back firing through the exhaust. So I checked timing and gave it a complete tune-up (plugs, wires, cap, rotor, PCV). Checked timing again. All checked good. Purrs like a kitten minus the popping in the exhaust.

My father and I think that it is a baffle in the muffler come loose and slapping the inside of the muffler. But, I am not convinced. I need to replace the exhaust anyways and have a set of 40 series flows for it. But, before I spend the money to do that, is there a way to check to make sure it isnt a backfire? Suspect maybe a burnt valve or some carbon caked up somewhere allowing fuel into the exhaust.