slowmo on 06 May 2010
It's clogged with dust as others have said. The dust stops air flowing through the heatsinks to cool it so it gets hotter and hence the fans spin faster. Removing the dust means it will run cooler and hence the fans don;t need to spin as fast. If its still doing the same after cleaning then you might want to consider getting rid as the YLOD is more than likely not far away.
