It's most likely something called crepitus, it's a form of arthritis. Heidi has this problem in her wrists and I guess it's very common. It's not usually painfull. My vet told me to give her Esther-C for it and the noise went away fairly quickly. I give her 3000mgs. per day. This topic comes up really often on here.
Here is some info on it
http://arthritis.about.com/od/arthritislearnthebasics/g/crepitus.htm
Typically, dogs that have hip dysplasia also have crepitus. I'm not saying that your dog has hip dysplasia or anything, just keep an eye for problems.
Here is info about symptoms of hip problems.
Clinical Signs:
Decreased activity; difficulty rising; rear limb lameness; reluctance to use stairs, particularly to go up; reluctance to jump or stand on hind limbs; swaggering gait; bunny-hopping gait; pain from manipulation of the hip(s); decreased range-of-motion in the hips; crepitus in the hip joint; positive Ortolani sign; positive Barden’s maneuver; subluxation or complete luxation.
Symptoms:
Less energy and movement; difficulty rising; lameness in the back legs; reluctance to use stairs (particularly to go up); reluctance to jump or stand on hind limbs; swaggering gait, bunny-hopping gait; soreness after lying down; soreness after heavy exercise.