Why does my German Shepard's Hips pop? Please Answer.?

He is a 8 year old german Shepard long haired. It is only when he runs like there trot and lope gaits. He doesn't limp or run funny, doesn't seem to be in pain i can play with his hips like pet him and he leans into it. You can hear them pop and if you put his hands on his hips when he runs you can feel it. also what is the best gait to tell if your dog is limping.