How do I get my fan to stop sounding like a jet ready for take off?

Ever since I've had my desktop, the fan has been really loud. It's starting to bother me more and more lately, so I tried looking up what to do and somebody said to enable smart fan in bios mode, but the problem is I don't have that option in bios mode. The model of the computer I have is Aspire XC-1660G. I've opened it up and it's not dirty or anything, so I'm really confused. If anyone can help I'd appreciate it!

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