I have a grinding sound in my knee with no pain... Suggestions?

I am only 18 years old, so I am a bit weary on the suggested 'arthritis' suggestions. I know that it is possible but I have experienced no pain at any time with my knee. It happens to only occur in my right knee. As I walk up stairs or stand on only my right leg and crouch on my knee, it creates a very audible cracking sound. However, it makes different sounds as I bend the knee than when straightening it out. When crouching down, it grinds. When standing straight back up, it grinds and seems to pop into place when I have it fully straightened. I do tend to stand with my knees hyper-extended, but as previously stated.. I have never felt any pain. I know that I need to see a doctor, but I would like to hear a few suggestions first. I am very short, tending to sit with my legs crossed, angling under my chair.. I also have shin splints if that is any concern in reasoning.