Why do I have knee pain?

I am 17 and male 170lbs 6 foot tall and little body fat. I use to run all the time but when i quit for a few years to sit a chair and play halo 2 all the time about two years into it my knees hurt when they were bend at 90 degrees or more only when i was sitting or kneeling down. When bent they would constantly build up pain until i stood up or bent them straight out. Then it would hurt tremendously for a few seconds but then the pain started to fade away. It cured it almost. Until i sat down for a few minutes again. It would hurt my knees to life my weight up from the ground if kneeling and when i lift myself up you hear a bunch of crackling. The crackling doesnt hurt at all, just sounds discusting. Strange thing is though is that i can jump run, roll, summersult, leap, skip, sprint, jog without them hurting at all. before winter came i was running 5miles a day and it didnt hurt them but it seemed to make them a lil stronger. So i know that my knees are not hurt by excersize. Could it have been the countless days sitting in a chair for four years of playing halo 2. Also, the first two years of the knee pain sometimes i would accidently bend my leg out from the stretch like from riding a bike, you would here a snap and they wouldnt hurt again for months. Now i cant seem to get that to happen. Like i said though i can kick,run ect and they are fine and strong, just when bent does there seem to be a problem. I also lay my elbows on my knees when playing games so that could be a factor. I know i need to see a doctor and ect but what i want to know for the mean while is what could it be, what could the possibilities of it be from what you have read? THank you. Im not overweight whatsoever.