6 months ago I bought a 13 inch Asus U31S laptop. It's perfect for school work and surfing the web. Lately though I have been experiencing some really annoying fan noise coming directly from under the laptop. So it's not from the fan at the side of the laptop. It also only happens when the fan turns off. Whenever the fan is on it's perfectly fine and nothing bothers me, it's when it turns off that it makes this really loud mechanical sound (as a motorbike turning its engines off) which even makes the laptop vibrate a bit. It also only happens when the laptop is horizontally placed. Sometimes I lay in bed and keep it in a somewhat vertical position and it doesn't make the sound.
I couldn't find anything about this on Goggle so hopefully some of you can help. Does anyone here have any knowledge about what this could be? What exactly is located there, is it the CPU or the GPU fan? How would I go around fixing this?
Edit - Here's a picture of the fan which I suspect is giving me trouble: http://i.imgur.com/qvP2z.jpg