the dust gets stuck around the fans and on heatsinks, forcing the fan to run harder to keep the same speed.
most of the noise would most likely be coming from your cpu fan, mines getting noisey at the moment, and i noticed the temps are upa bit from normal. so when i get time, i need to do some cleaning!
P4 3.00Ghz/HT 1Mb Prescott OC\'ed to 3.50Ghz
1024Mb DDR 400 PC3200 [2x512mb]
Western Digital 80Gb 7200rpm
Seagate Barracuda 160Gb 7200rpm
Seagate Barracuda 200Gb 7200rpm
Nvidia Geforce 6200 256Mb
LiteOn 12x DVD + R/RW