Yeah, I had a period of a few weeks post-surgery with a lot of sternum popping, much like the tectonic plate shifting that Dianne is referring to. My heart buddies here on this site, including Hank, assured me at the time that it was normal and would clear up, which it did. Kind of a funny, somewhat disconcerting feeling but not painful and not problematic as far as I understand.
Of course, I think it is a sign that the two sides of the breastbone have not yet fused back together and thus are still quite vulnerable to mishap if you are not careful. When I was in the CICU my first day after surgery, there was a guy in there recovering from having his sternum repaired after he had not respected his sternum precautions, thus breaking his wires and really messing himself up. You do not want to go there! To me, what he was going through was far worse than the valve surgery itself.
So, don't worry about the popping in itself but do be very careful about adhering to your sternum precautions until all pain, popping, etc. have ceased and your surgeon clears you for lifting, driving, etc.
Sounds like you are well on your way to a super recovery and you will be amazed at the how quickly you will surpass all the milestones along the way. Of course, there may still be some bumps on the road and please take full advantage of this group of heart buddies to help you get through them. They are the greatest and have been a huge source of help and encouragement for me.