Could be your CPU fan engaging in a higher speed to keep cool when increasing the workload on it. Sometimes the bearings wear out in them over time and they can become quite noisy. Eventually it would need replacement if that's the case. It may also be blocked fins on the CPU heatsink where the fan passes air over it. Those fins get blocked and therefore the CPU can't cool properly and as a result, the system cranks the CPU fan speed to its highest setting as a countermeasure against the additional heat. Either way, it's probably time for it to take a trip to the PC shop for some maintenance and/or replacement of a CPU fan.
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