It sounds like something (it could be a flexor tendon or anything) is pulling on the SI area which is popping because the SI area is gummed up by tight muscles of some sort.
Er, SI joint is more posterior? Is that or not? If it's between the hip and iliac crest it could be hip joint.
Both hips and Si joint tend to get gummed up by the same tight muscles cause a bunch of them cross is especially the glutes