I think I have the same issue, and would also love to hear some advice. Personally, I think it may be the serratus posterior superior muscle, but am not sure; I can massage it, or set about it with the tennis ball, but it only gives very temporary relief. Do you get tightness or numbness around your same-side pectoral, teres major and arm? I do, myself, and I would wager that it's all inter-related. I have been to see both a physio (free on NHS) and a chiro (which I paid for) but as I'm dealing with a couple of different issues which need resolved, this one in particular was overlooked by both.