Get it looked at, had the same problem at your age with my right knee, it also used to "lock out" allot which was rather painful. Eventually when i was 24 the cruciate snapped on a rapid decent of a nice hill in Long Valley. Having said that there is now no prob's and don't notice the absence of said ligament.
Either way take the general advice and get to the doc. Only thing to consider, is most GP's fix colds, not musculo-skeletal injuries so if you are even remotely unhappy with what the doc says, seek someone else out, like a physio.
Get it seen to, quick. Clicking and crunching usually shows cartrilidge damage, I think.
Osgoods Schlatters is easily identifiable because under your kneecap you will have a lovley big bump, do you have this? If you answered yes then you probaly have osgoods schlatters, which goes when you have grown up, either way get itseen too.
If it is happening a lot & is painful - GO TO YOUR GP & GET A REFERRAL TO AN ORTHOPETIC SURGEON!!! (I wish I had when I was your age!!) also I'd advise you to keep a diary of what your doing (eg exercise types etc), when you get the pain & where it is - it will help the GP & Surgeon.