Depends. Sometimes it's just like clicking your knuckles or something, which is a little painful if you do it too hard. Though a couple of weeks ago for some reason it decided that it was going to be hard to move my jaw about, which has never happened before. Oh well, it doesn't really affect me.
You should go to a physical therapist. I had the same problem and got it fixed with a few exercises over the course of a few months. Get it fixed now because when it continues your hips and ankles will also start going out of alignment as well. Let me tell you, having all three out at once is worse than just one knee.