Q#1 Does your 2012 Mini have it's original hard drive or an SSD?
Q#2 When was the last time the fan was removed and cleaned?
I have the same model in which I installed an SSD, cleaned the fan and at the same time repasted the CPU & GPU.
Must admit I've rarely heard the fan on this machine. Often on many Apple products the CPU is poorly pasted from new and on all my Macs (desktops and laptops) I make a point of cleaning the fan & repasting the CPU/GPU every 4-5 years (this can be a nightmare on early Powerbooks) but tear-down on a Mac Mini is very straight forward.