Snapping joints?

I'm 31 & in pretty good shape. I've had fun in my youth & had more then a few injuries including broken bones & dislocated shoulders. My question is why do my joints snap, crackle & pop often to the point of hurting. My knees are probably the worse & while the pain isn't unbearible it is annoying to say the least. It is funny when people around me especially those who haven't heard them snap before hear for the first time their reaction is just to funny most freak & ask what that noise was. I was very active in sports & being adventurious but it just seems like even simple things like getting up off the couch or out of bed my joints shouldnt noises like they do. I could understand if I did something extreme & while I have had more then a few bad injuries like a fast ball to the knees it just doesnt seem they should be this bad already.