pain and "popping" on outside of knee (youtube video enclosed)

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.