I've had this happen to me for years. At first it freaked me out. I had no pain, just the popping in the sternum when I would do dips or ncline bench. It eventually went away, but after a few lay-offs from lifting, it would always come back. A doc told told me it is very normal, that its an over-abudance of cartilage in the sterum that separates. He said some people have, some don't, and to not worry about it as long as it wasn't painfull.
Just some 411 on my experience with it.