Can you describe the noise? Click, tap, chatter, whistle?
I am not familiar with the 318 engine, but the most common noises on the six cyl engines are the tensioners (actually the tensioner pulleys) on the serpentine belts. He removed the belts, so disturbing old tensioners may have made one start to chatter.
I recently replaced the water pump on the wife's 330, and it developed a sort of odd whistle or moaning sound. I suspect that I created an air leak, perhaps in the rubber hood over the throttle body. Those get old and brittle and crack eventually. The car runs perfectly so I have not spent much time looking for the problem yet.