The BIOS should have an option to control the fans speed... it may be set to a predetermined speed or autocontrol it, and run the fans full speed if the system tends to overheat... or it may not have the speed control, depends on how old the system is and if the BIOS was not updated.

Also, case fans tend to vibrate an make noise if they are not secured tight enough on the case.

A couple of applications you can use to control the fan speeds

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