I've had this for ages and it's begun to annoy me. There is no pain, at all, associated with it. It feels very bone on bone-ish. A loud "click" sound when I bring my leg down. If I'm laying on the floor, standing or sitting, it pops on anything where I'm bring my femur in alignment with my spine/body (so imagine leg raises, the portion where the knee is at my chest, and is now coming back down). Doesn't happen if I'm running, deadlifting, squatting or what have you. It clicks when the femur/leg is almost completely straight (as if I'm standing). This is for both legs. What is causing this?
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.
Front hurdler stretch done religiously morning and night gave me some relief after a couple months. Physical therapy type exercises are also mentioned by doctors, but couldn't tell you which ones. It was very annoying, like cracking knuckles, sometimes you can't resist doing it.
general consensus is that if there isn't any pain then it's nothing to worry about. It may be annoying but there's no definite reason for why people get it. Leading suspect right now is leg length differences causes tension, muscle and nerve timing imbalances. Realize that if that's the case you don't want to get rid of it because trying to balance those out will be detrimental since you can't change the length of your legs.