I sometimes get sternum pain from getting punched there. For me the "walrus stretch/cat stretch (same as the dand exercise from Hindu wrestling, similar to a dive bomber pushup, except turned into a stretch)" fixes things up. But since you said stretching just causes a crackling noise, I'm not sure you should try that. You said you worked out a lot, maybe you are working out so hard that your body is destroying itself. My physical education teacher from 9th grade had a Pac 4 record in rugby, could sprint faster than any kid in our class (which is amazing since my school has a really rigorous sports program that has taken us to several state championships), and bench press over 250 lbs (he could do 300lbs on a machine, so I'm guessing 250 without machine). Recently, injuries have popped up in both his rotator cuff and his ankle, which both required surgery and both he might never fully recover from. Maybe you should ease back of working out, especially stuff that is taxing on your bones like weight lifting or sports involving contact. One last thing, putting heat packs on your sternum may help since heat promotes more blood to flow to your sternum, icing your sternum might help unswell any sort of bone swelling there. That's all I got to say, try not to worry. In 8th grade I bruised my middle finger knuckle from punching a metal pole, afterwards any sort of pressure applied to my knuckle would produce a cold shooting sensation. After 2 years of punching a bucket of sand and using heat packs, my knuckle made a full recovery.
