I don't know all the specifics, and by no means am I an expert, but I do know how you feel because I have a lot of problems with grinding under and all around my knee cap. I have a bad case of mal-tracking, so that's a big part of the problem right there. Have you noticed malalignment of your knee cap? I guess that's where I'd start. I'm sure there are other causes, but that's naturally the first thing I think of due to my situation.
Personally, I'd recommend talking to an OS or someone equally knowledgable about knees, because things could possibly get worse if not treated. Also, there's absolutely nothing wrong with wanting to know what's going on, you have that right. Good luck, and keep us posted!