I actually just went through this with my white unibody macbook. It sounded like metal tapping on metal clicking when i touched the trackpad, and it was very clearly not a normal noise. I tried to put up with it, but it was driving me crazy.
I ended up taking it in a few times, and when no one could figure out what it was they actually replaced my whole machine.
I highly recommend that you bring it into be looked at, because its the best way to get answers (and results!). I remember being really frustrated when I was having an issue, because there was no information on it in any forums, so I just wanted to let you know that it has happened to atleast one other machine!