Have you ever dropped it at all? Mine had a corner dented in a TINY bit but had it replaced under Applecare and since then I have same thing. If you press down on case top a bit you may see noise gets louder a bit. I think its something to do with hardrive turning or like stated the fan pressing on something. Not sure how a piece of paper would find its way in there, but ya never know.

One of these days I will bring it in to have them take a listen to it and report back. Its certainly not as loud as a lawnmower though so you may have it worse than me!