Knocking/Popping Sound.
 

It runs!!!!!! (My 91 NA that hasn't been running for several months)

but......

Now there's a knocking/popping sound that seems to be coming from the rear of the engine. Outside of the car it sounds a lot like highly pressurized gases popping out of something. I can also see and feel a puff of what looks and smells like exhaust gases come up from the back of the engine when I rev the engine up. Inside the car it sounds a little like a tiny knocking noise.

Can anybody speculate as to what they think the sound might be? I'm pretty sure that this the sound that my mechanic identified as "rod knock" so I guess it might sound like that if anyone has heard it. I thought it might be an exhaust gas return line with a leak in it. Would that make a popping noise?

Thanks as usual for all the input.

only guessing, but I might say a broken or loose spark plug?

or loose exhaust manifold?