What is the popping sound in my sons chest?

Hey I'm a EMT from Ga and my son is 6 yrs. old and i was listening to his breath sounds after he had got choked on a pc of candy about a month ago and to make sure he was ok and when i was listening thru stethoscope i felt something popping on his right lower rib cage about the 4th and 5th intercostal space. you could hear it and feel it moving and popping when he takes a deep breath in and breaths out. it reminded me of a flail segment sort of. I took him to dr. the next day and she has never seen it or heard it before in any of her pts. She sent him for xrays and it didnt show anything and she said it could be cartilage. He hasnt had any problems until 2 days ago and he started hurting there and in his chest especially when he takes deep breath or sneezes or coughs and he just cried with it and was in pain. I took him back to dr. and we have an appt. with elgeston in Atl. monday. Have you ever heard of this in another child or have any ideas what it could be? It is weird cause I have never seen it before and especially you can feel it when he breaths in and exhales. Thanks