Cracking Knee

I think I have some obsession of cracking my knee every 5 minutes, I need to extend my knee to crack it, especially when I'm sitting down for long periods of time. It's a really satisfying feeling. I'm 16, I'm a track runner and as far as I know I don't have a knee problem. It never really cracks when I'm not trying to do it, only when I try to. I sort of get a feeling in my leg when i know a crack is coming

1. Is it healthy doing this?
2. What's actually going on inside my knee when it cracks.
3. What makes it crack( For ex) after a long car ride it tends to feel the best.