I have two possibilities for you, I have experienced both.
1: When i got my unibody macbook i played about with opening and closing the latch to the battery an HDD because i was impressed with the latch system and although used for serious work it felt like a new toy at first. Obviously this meant it got more use than intended and it must have bent open the fixed/non mechanical catches, (at the front edge of the notebook, not the middle) to the door developed a little play, and therefore a creak. I carefully bent these bits of aluminium to achieve the original tight fit and this cured it. Edit: Just noticed you have the newer sealed design. Possibly slightly loose screws????
2: While open the macbook got nocked off the sofa. No visual signs of damage, but I could now hear a faint creak from what I thought was the right hinge. I assumed I had jarred and slightly damaged the hinge. A few weeks later I took a closer look and found that the bottom right corner of the screen must have slightly bent inwards towards the power button by a fraction of a mm, but this had been enough for it to rub against the main unibody of the laptop when opening and closing the lid. Again, very carefully bending the bottom right corner of the screen back slightly cured it.
The slight bending in both cases was so minute that they couldn't even be seen. That goes to show the fine tolerances and finish these laptops have been designed to. It took at least 10 minutes of close inspection to work out the problem, almost by blind but educated guesswork. Although aluminium will bend i still believe the unibody to be the best built and solid laptops going. One minor gripe for somebody who never uses ethernet, is that the port is so big that the aluminium around it has to be impossibly thin. In all the arguments about the quality and strength of the unibodys, this seems to be the area that is damaged and bends when dropped. Obviously because everyone is like me and never uses ethernet

ethernet should be removed from apple's unibodys.