|The CPU Fan are made to spin till the computer shutdowns, its just that sometimes you don't notice its spinning because of its low spinning speed which generates no noise.|
As you said its keep spinning that means its spinning at more speed than before. The causes for it are something is hogging you CPU power & CPU heats up, so to lower the CPU temperature the FAN spins at its max speed making noise, check Task manager for CPU consumption.
The another problem can be someone has messed with your BIOS setting and configure it to spin at max spin all the time.
Also check if your CPU fan is plugged where it should be and not in system FAN pins or it will keep spinning at max speed.
NOTE : A CPU fan should spin all the time as long as your computer is on otherwise it will end up burning your CPU.
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