Engine Noise

I have a 72 Nova that was my brother-n-laws. He was driven the car for a year or two. It was setting for a month or so. He got in and drove it across town when he noticed a noise from the engine compartment. I am not sure I can describe this noise but here is my attempt at it.

When the car is running at idle it sounds like a loud, very loud, popping noise. It sounds like is it is coming from the drivers side of the engine. He said that when the noise started it went away when he got up to highway speeds. I have never heard this noise. It is so loud that I was thinking if it was an internal engine part it would have broken already. But I am not an expert. So any help would be great.

Also can you all recommend any books for the Nova?

if its only at idle and goes away when your at cruse speeds its more than likely an exhaust leak at the manifold gasket buy the down pipe or at the head, cheap easy fix