The common causes for such is heat buildup, or the the fan is failing, or the fan is supposed to do that..
So, start by removing the computer cover and using compressed air to clean out the dust from around the processor, the heatsink, the power supply, and all vents and fans.. If that doesn't help, try replacing the fan.. And because there could be an issue with improper cooling from the heatsink, reapplication of the thermal paste will also help most.
Hope this helps.
The fan on my Dell Optiplex GX620 PC kicks off and never stops until I turn the PC off. When I restart the PC it is not running but will eventually start up and run for ever. What would cause this and how do I fix it?