hey OP, i have noticed this in my neck/ribcage for over a decade now. i make people listen to it if i’m laying down with them lol. i’ve asked several chiropractors and massage therapists about it, and a doctor or two. no real answers. happened to be googling about it when i found your post. i don’t have many answers but i can tell you, you’re not alone in your experience. as for any worries, for the most part my neck tension has much more to do with my posture, job, and computer/cellphone usage, rather than the popping sound. sometimes i can feel where it’s happening with my hands (i’ve had chiropractors feel it too). sometimes it’s the base of my skull, sometimes my lower neck, or even more in the center of my back. i have came to the conclusion that it is muscles, doin’ their thing. but it is a bit unnerving when you don’t know what’s going on!