Everytime I straighten my leg my knee cracks and pops. No pain. I hurt my knee when I was 12 jumping off my house. Never got it looked at.?

Should be OK. We used think that knuckle-cracking is bad and one may develop arthritis..There is no such evidence. As long as you are not having pain or instability in your knee, popping/crackling by itself is likely harmless.....
Cartiage. May have some cartilage damage. Other possibility is the fat pad behind the patellar tendon.