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.

Good Question I have a 40 gig fat, thats close to 3 years old, and it is so dam quiet, I can never even hear the fan, and it gets crazy use, atleast 4-6 hours a day, and more on weekends, Its the primary DVD player , and Bluray, the only thing I do is clean it, I have never used a vacume on it but ounce a week I clean all the duct off , and dust the vents also, I keep mine flat down, not in the air, also some people have there PS3 on top of there cable box, or A/V receiver that gets really hot, and they dont realize it, so its giving of lots of extra heat to the PS3 because its sitting on something that is hot to the touch, keep your console on a shelf of its own with plenty of room for air to circulate, this is what I do for my PS3 , its about 3 years old, never ah problem, and even when the fan kicks on , you really can;t even hear it, so maybe try that.

Don't use a vacuum on computerized equipment as the vacuum creates static electricity that could damage various components. Buy a can of compressed air, shake it, hold it upright and fire it in the holez.

no no, the same thing is happening to, me it all started after an update, i dont remenber wich, but i also have the MGS4 80 with BC and it has gotten louder lately, it wants like that in january and its still loud even if i have been gaming for a few minutes.

I got a 60gb backwards compatible model and mine gets loud too after it runs for a while. I don't know if there are any solutions to the problem.

it's dust, or simply thermal paste it's gone.

need probably clean, also playing in a cold enviroment helps,

Alot of the problem might come from it having dust building up in the system.

Need to clean it out.

Yeah, mine only gets real loud in the summer, as I guess it's the heat that's causing the big problem here. I generally dust it once a week and run a vacuum on the vents about twice a month.

Thermal paste probably dried out. Replace it while you can. It can't be the dust, when I cracked open my PS3 I was expecting to find mountains of dust, since everything else in my room usually get's dusty but there was barely any in the PS3.