Noisy Computer Fan (specific issue)?

My grandfather has been complaining about a noisy computer fan. One of his computer-knowledgeable friends told him that it was the CPU fan (the one under the green cover in the Dell 4600). Three to four years ago, we ordered two Dells 4600s. One of them is in my house and other is my grandfathers. The configurations are essentially the same with the exception, I have a x1600 ATI and he has a 5200fx Nvidia, and I have a sound card. I have a 2.4 GHz and he has a 2.8GHz Pent 4. Now I don't know if the sound was always this loud but it is noticably louder than on my PC. What I've noticed is that if I take off the PC cover and open up the green CPU cover while the computer is resting on its side, the fan does not make noise. As soon as I put it right side up, close the green cover, and close the PC cover, the fan gradually increases noisewise.