My sternum pops once in a while, usually after a day or two of hard work/exorsize, I just get a mild generaly tight feeling and then a few minutes or hours later I will inhale deeply streatch my arms out and puff up my chest, then POW!, it has made people across the room jump. I feel great after, no more tightness or any thing.

Maybe it's built up CO2 being released, I've heared that is what makes other joints crack and pop. I find it odd though as it isn't really what one would consider to be a mobile joint.