My massage therapist described it that same way - tendons and ligaments sliding over joints.

Since I've started doing yoga, I notice every class begins with different popping and clicking sounds . . . and they change depending on the week or month. Some of the sounds that I thought were a permanant part of me have now gone away.

As long as there is no pain, I don't think it is a problem. Yoga class might be an interesting experiment.

Sandy