John, Mike.

Crepitus it absolutely is.

I have recently being diagnosed with Chondromalacia Patellae which is effectively wear and tear of the inside of the kneecap (Patella). This involves very loud crunching noise. see more here

The clicking is caused by rough edges in the knee joint crunching over each other, as opposed to sliding smoothly over each other.

The condition is caused by mainly muscle imbalance, compounded by the amount of sport I play, teased by a few accidents, and completed by AS. Interestingly enough the specialist that saw me was amazed at the little amount of damage caused by the ferocity of AS I experienced as a kid. Which is good. Am doing special exercises and having my knees 'taped' to try and correct the issue.

Furthermore I have had extreme click-clacking in my neck since early 20's. This is also classed as crepitus, although the cause is thought to be slightly different here. Liagments can suffer inflammation too as a result of AS. And when they do they can become a little 'wooden' and lose a little of their elasticity. As a result, where they would normally slide smoothly over muscle and other tissue, they get caught, and then release with a bit of a snap or whipcrack. This is the case in my neck.

Also, as George mentioned, where there is some fusion present, close proximity of bone where there shouldn't be will cause noises when you move as well.

For my neck, I just stretch it all as much as I can to keep the ligaments and muscles as flexible as they can be... and put up with the noises.

Although there is an amusement factor when my neck does a really loud one at work to the horror of my work colleagues sitting 6 feet away.

Thankfully this is rare.

I hope this helps
Cheers
Jo