My friend you are 6'8" I am 6'7" with cracky knees since the 7th grade. We have something in common.
For me when i hit my growth spurt and skyrocketed the growth plates around my knees grew at differnt rates. As a result my knee caps glide insted of vertical at a slight angle thus irritating the tissue behind it. This was painful.
Long story short I gave up football and wrestling because of the pain. Started powerlifting and as soon as the muscle built up in my knees no more pain and minimal cracking.
I would bet your knee caps glide at a slight angle too.. Strengthen my friend