Spring cleaning and my niece's growing and therefore growing demand for toys led to the partial emptying of the attic at home. My room is right across from the attic, with the computer aimed right at the door. Over the several days of the door open and dust being wiped and blown off boxes, my main fan kept on doing its job of drawing air in the front and out the back. Along with all the dust. This put a nice thick coating on everything from my video card to my heat sink.
In college I had classes in a room down the hall from one of the wind tunnels they used for experiments. It was loud. My computer was louder. Yep, turns out dust interferes with radiation (however this does not make radioactive dust safe) of heat, which meant that the fan had to run faster to try to speed things up. Fortunately, Intel in its finite wisdom decided to make the Pentium 4 throttle its speed if it got too hot, so my computer just ran loud and slow rather than quiet and melted.
But I wasn't going to stand for this. If there's one thing I hate, it's unproductive loud things blowing hot air, which is why I like politics. So I opened up my computer and took out anything that wasn't screwed down. And some things that were, thanks to my screwdriver. That included the video card, RAM, hard drive, other hard drive, main fan, heat sink, and for the hell of it, the CPU itself. Then I did the logical thing: carefully dusted them with canned air in order to avoid damage to the delicate electronics.
Nah, I'm kidding. I used an easily-frayed paper towel and a small bowl of water which I left on the floor near the computer. Sometimes I think I'm trying to make an excuse to buy a new computer. Which I almost needed. After I stuck it all back together it didn't work. Oops. So I rearranged things and restarted and swore a bit. Then it worked better.
But let me tell you about this stuff called thermal cement. You know how oven mitts keep you from getting burned, but if you pour water on them they instead boil your hands off? It's like the water, except instead of your hands it's a heat sink and instead of a pan of delicious chocolate brownies it's a CPU and instead of being a bad thing it's a good thing. And the oven mitts are the poor conductive properties of air. So I replaced my dust-insulated heat sink with an air-insulated processor, thanks to not using enough thermal cement. Or originally, not using a fresh coat. Then I gave a nice thick (but not too thick) layer, stuck it back in place, and viola (not to be confused with a violin), my computer ran quietly.
It's nice when running a game doesn't cause my fan to out-drone political commentators, since I often have them playing in the background while I play Civilization.
The moral of the story is: you can't trust the system. To run quietly without proper cleaning.