If there is no pain, crunchy/grating sensations, or movement restriction involved, the most likely explanation is that the sound is caused by gas escaping from the joint. This is what makes the noise when you pop your knuckles, or a chiropractor "cracks" your neck. It's perfectly normal, and not a sign of any problem.
Here's an a more detailed explanation: www.wisegeek.com/what-exactly-is-happening
Hope this helps.