The only thing I can think of without giving it too much thought is Snapping Scapula Syndrome, (1) when the soft tissues between the scapula and chest wall become inflamed or fibroused or (2) the mucle have atrophied and the scapula is riding more closely to the rib cage and bumps or rubs on the ribs during movement. Also, when a possibly fractured is misaligned rib or scapula isn't lined up appropriately it can cause a grinding or snapping asss the scapula moves over the thoracic wall.
You may try:
1. strengthening serratus anterior, infraspinatus.
2. Postural re-education
3. Scapular release of infraspinatus especially since you say yourself it is fibroused.
4. Test thoracic mobility
There are quite a few other things but this is it for now since i've gotta head off to work. Peace chief