There's a couple of threads over on AVForums on this issue, it seems that some folks have virtually silent slim models and others are having the noisy fan issue.
AVForums Thread
The first slim I got (which I had to take back due to no HDMI output) was a noisy brute, but the one I have now is silent except the occasionally noisy disc access.
As Pabs has said dust can be an issue, and from the AV thread the location of the console seems to make a difference too.
If the noise is irritating while you're playing I'd be inclined to take it back to Game.
Hope you get it sorted
