I've never felt a pop either. My experience is much like JJs with the muscles located between the ribs. I also get esophageal spasms that feel much like a heart attack.
Hi there, I've been experiencing intercostal muscle spasms (hug) which are the muscles located between each rib, since 08 but i've never felt a pop as such and if by palpable your meaning you can feel something like a lump etc. then definitely not. The hug for me is 1/2 my ribcage visibly contracting and locking down for awhile, then releasing before it repeats, it can last hours to days.
Sorry but it doesnt really sound like the MS Hug that we talk about, the MS hug is generally described as a banding feeling fully or partially around your torso, intercostal muscle spasm (rib cage) or below the ribcage oops forgotten the muscle group that spasms when its below the ribs. I have read something about thyroid issues having a similar sx, though i can't tell you anything about it, but it might be worth googling to see.
Cheers........JJ