Hi, towelhorse here I too have the clunking as I adopt certain postures. I showed my treating doctor how, if I moved my shoulder blade there was an audible clunk. If this happened several times my arm would become heavy and then I would get strange sensations in my arm and hand. The more often it occurred the worse all my TOS symptoms would become. At its worst my fingers curled up and I could not sign my signature. A couple of years later I was diagnosed with partial long thoracic nerve palsy. The serratus anterior weakness caused muscle imbalances around my scapula. as a consequence I had pec. Minor tightness when my serratus anterior weakened. I am sure that the clunking was the scapula bursa grating against my 2nd rib and compromising the long thoracic nerve. I suggest that when it occurs that you try to stop doing what you are doing at the time. My significant improvement (TOS and scapula stability) has only come about because I have either strapped or used a shoulder brace to stop the snapping scapula. if you search snapping scapula you will see what I mean.
http://www.drkelly.us/snapping%20scapula.asp
I am the only one on the planet who believes this, but everything which makes this rib, nerve and scapula problem worse makes TOS worse. For example, chest breathing, putting on weight around the abdomen which places greater pressure on the diaphragm, bending down to the ground, sitting, reaching upwards (stabilizing your scapula), driving, reading the newspaper, sleeping.
Put pressure on this nerve—lose scapula stability-pec. Minor muscle over compensates---hand tingles---trapezius and rhomboids over compensate—thoracic spine tightness—T4 syndrome---layers of pain—

Its weird, the PTs try to teach TOSers to abdomen breathe and surgeons remove ribs (unfortunately not the 2nd rib) and people have scalenectomies (the long thoracic nerve pierces the scalenus medius) I reckon they know that the ribs and scalenes are part of the problem but they don’t know why.

If you allow the clunking to occur too often there will be a burning sensation under the scapula. this is not good. It takes a long time to improve from this position, especially if you are expected to attend rehabilitation sessions. I am afraid rehabilitation waits for no one. Regards towelhorse