mine is a snapping as the tendon moves across the bone. Sometimes it hurts so bad, makes me wonna cry. Going up stairs or getting up from a low chair cause the knee to have to move through a range of motion with pressure on it. When the range of motion is 45-90degrees (or even less when flared up), causes tendon to snap and pain is very bad. It causes my knee to sublux to the lateral side. Because of that, for awhile I used to think my knee "popped" out of place when going upstairs, etc. After living with it for 15 years, I realized it was a tendon above my knee cap that snaps as it crosses a bone during flexion of the knee.