I have the same issue, and have been trying to work out what it is. From the research I've done, I think it's likely to be what is referred to as "Snapping Hip Syndrome" (http://emedicine.medscape.com/article/87659-overview). It seems to be caused by either the IT Band gliding over the greater trochanter, or by the Iliopsoas gliding over the iliopectineal line (a part of the pelvis.) A lot of the time it's painless, and the feeling is generally that if it's not causing pain then it's not an issue; however, I personally would worry that over time it may lead to irritation or bursitis, so would also like to resolve the issue. I've been working forever on hip mobility and overally flexibility, so don't know what causes it to persist.