I really don't know much about cats Kirsten but I remember one horse I had that would crack and creak in her ankles when I would take her out, she did this for about 10 minutes and I was convinced she had arthritis. The vet came over and he found it funny....he listened and checked her good and turned out that she had oxygen that was locking in her ankles and it would work it's way out after some walking. After she had been with me for a few months it went away and I never heard it again.
I hope it's something as simple as that for your cat. If it is arthritis I'm sure they have great meds like they do for dogs.