Crunching Knee

One of my knees makes a crunching sound whenever I move it or walk up stairs. There is no pain. I fell on it about 5 years ago and since then it has been making this noise. It gets worse in hot weather and has seized completely a few times in both bent and straight positions and I havent been able to move it for several hours.
Oh Im only 22 and it fairly good health otherwise. I always drank a lot of milk and never broke a bone or needed a filling in my teeth so I dont think its a bone issue