I sent information from a respected medical site about Marfan syndrome by private message right after I posted here. I mentioned it not to say she has it, but because it can have affects on the skeleton, including the sternum, so she could look if she cared to and see if she thought any of the description sounded anything like her own situation, etc..
Hi Stph, I would google marfin syndrome, I did and cant see how it relates?
I just think you need to get out and do some exercise, get on a good healthy diet, and you may find then it will go away, I think the popping noise your getting is from carbon dioxide, that builds up when relaxing.
So just get out and get healthy.
Good Luck
Have you gotten it x-rayed?
Have you ever heard of Marfan Syndrome? If not, you might look at a private message from me to see if you have symptoms that would prompt you to ask a doctor about it.