If I could give a one word answer it would be dust. Dust throws the fan out of balance and causes it to make more noise than usual it also decreases the cooling efficiency of the computer so the fan runs harder for longer .To clean it you can do it one of two ways . one is to take the computer apart and physically clean the fan this will yield the best result . Or you can by a can of compressed air from your local computer store and blow air through the vents both ways works best one way for a while then through the exhaust. If you have access to an air compressor you can use that as well as long as you dont hit it with too much pressure. Hope this helps