I can crack my right hip but I wouldn't recommend it to anyone else - it feels great at first but I think I may have done some permanent damage - I get long periods of low level pain in that hip now and find it difficult to lie flat comfortably.
PJ - are you sure you're cracking your hip and not your lower back? My hip was never very loud (compared to say, kunckles), more a dull thud.
An article titled "Knuckle cracking: key to a better life, or the road to a living h**l?" used to be on the cambridge university first aid society website. It has since disappeared but is still available in web archive:
It offers a useful summary of some of the (pretty limited) scientific research into knuckle cracking. You may want to link to it from an FAQ section or reproduce it on the site (check with the author first).
I find that my whole back and hip is asymmetrical for cracking.
They used to be fairly even, twisting one way and then the other would produce a string of clicks down each side of my back, sometimes as far as my sacrum and hips. Now my right side always cracks very easily (normally in the hip or lumbar area) whilst I can almost never crack my left side.
I think I may have caused some laxity in the joints of the right lower back and hip - when I try to twist left it feels like I don't have enough leverage to crack the left side joints. Sometimes when I twist left, the right side joint will crack!
This worries me a little and I've been trying to stop cracking these joints, however it's currently so easy to crack my right lower back and hip that just twisting towards someone or turning over in bed can cause the joints to crack!