I am a 26 year old female, 5'7", 135lbs, no children, moderately active, no known trauma. For several years now I have had this uncomfortable feeling on my lower left rib cage. when I am laying on my right hand side I can feel a popping sensation with every expansion and contraction of my left lung. You can hold your hand on my rib cage and it actually feels like a small thumping from the outside. I also get the thumping sensation randomly when I'm slouched a little and turn my torso to the left a bit (say for instance if I am looking over my left shoulder to change lanes). The sensation seems to my more prominent when am laying down, especially if I have a full stomach, its not painful as much as it is uncomfortable and irritating. Every now and then there is a sharp pain in that region, but it is not that common (maybe 5-8 times per month).There is also a dull ache/tenderness a few times a month as well. I have asked my doctor about it and she seems clueless.