I have something similar that appears after activities such as squash or basketball. I feel a sharp pain just under my knee cap and feel the need to bend my knee 3 or 4 times until it clicks loudly and the pain subsides for a few hours. I have given up squash because of this injury, but it dosent appear after mountain biking, runing or similar activities. The pain can be very sharp and painful and usually lasts 3-4 weeks, its directly under the knee cap, sort of at the soft gap between knee cap and top of teh shin bone and I suspect its one of the ligaments.Doctor just said stop doing sports which seemed a cop out, and I havent found much on the internet thats relevant or similar. Fortunately mountain biking does not bring it on so ive learned to live with it.