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Welcome to the forums. ^_^
Do you mean the fan in the very top back, right next to the power plug? If you do, that's the Power Supply Fan. It vents air from the inside of the case, and ideally helps keep the PSU from overheating. If it's the one right below that, it's just a case fan (a dime a dozen, so to speak).
Anyway, if it's suddenly gotten loud, it could mean a few things. It could just have a lot of grime in it. You could (and if you've never done this, should) get a can of compressed air, take it outside when the weather is clear, and blast all the dust out.
If it sounds like its starting to grind or something, however, that could mean that it's approaching the end of its life. When the fan fails, the power supply itself usually follows shortly thereafter.
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