I have a Mid-2007 Macbook and since about a week ago, the fans have been making a sound after startup that sounds like they are slowly dying out. It sounds like the sound that fans make when they are slowly petering out after running for a while. Is this the result of dust build-up or something more serious? If so, how do I safely clean the fans? I really hope that it is the fans and not something more critical like the hard drive.