I found this - http://www.rehaborthopedicmedicine.com/noisy-joints.php
It says the popping, grinding and creaking is Nitrogen bubbles in the synovial fluid that get trapped then released during particular movements, and it's called crepitous.
You can look for more info on it, but the basic consensus is that if you don't have pain, it's normal. See an orthopedist if you want a a definite diagnosis. I probably would.