1970 Chevy Truck Engine Knock

My engine has recently developed a knocking noise on the drivers side of the engine. We pulled the valve cover to inspect the rocker arms, they looked okay. The noise is coming from the rear section. When the engine is idle it has a distinct knocking as if it's metal against metal. At certain times, the noise goes away(yet not very often), then returns . What is the most common issue for this noise, what should we be looking for? Next step?

Thanks,
Greg Owens