I had to open my Macbook to change hard drives today. I used one of these guides. http://eshop.macsales.com/installvideos/
While I had it open I thought of cleaning the dust. I used a dust blower and a vaccum but I could see the fan was still dirty. I deciced to remove it. It was just 3 screws, I didn't disconnect any wires, just moved it to the side. There I saw 1/4 inch thick dust on the grill that separates the fan from the external. Looked something like this 
Dust sticks on the grill like lint on a dryer and there is no way to remove it without removing the fan. Now that it's clean I can't hear the fan anymore unless I put my ear on the keyboard, plus it doesn't have to run all the time because when it runs it's much more effective.