what is the "popping" on the outside of my knee. (youtube video enclosed)?

[runninman]

What can be the cause of this "popping" on the outside of the knee when flexation occurs? Are ligaments rubbing against each other? If so, which ones? I Tried making an appt. with an orthopedic doctor, but his secretary said he couldn't help me. Pain associated with this had been gradually increasing over the past few months. I am trying this forum before continuing to go through the process of obtaining an mri, since it can be seen from the outside somewhat; and when I do see a doctor, I can have an idea of what's going on.  What is the best method of treatment? PT surgery, etc. ? Any feedback would be appreciated.