Is it louder under the car or under the hood? If its under the car then in most cases it will end up being a connecting rod bearing. If it seems about the same up top as the bottom then its most likley piston slap.

Both are comon on high miles motors. Most SB see this condition towards the end of their life cycle. First the noise starts just when its cold, then it takes longer and longer before it goes away till one day you notice it doen't go away. I have seen cars in normal (not hot rod) service last 40,000 mile after a noice started and most of the time repairs are made for piece of mind.

Course this all assumes we are talking a older motor. If we are talking a late model ya got to get if figured out. These tightly toleranced precision engines are not very forgiving. They will run strong for near 200,000 miles but once something goes wrong it goes quick.