Hello there ~ The popping sound that you hear is possibly the same mechanism as when you pop your knuckles. Yes, there are joints in your rib cage
too. If you feel like stress
may be adding to it, I suggest that you try relaxation and deep breathing exercises, as an alternative to cigarettes. It may also be helpful to get a chest X-ray just to clear any doubts about any rib-cage problems. If you are still not satisfied, please consult a doctor. Take care and have a stress-free day.