@matty wrote:
probably a faulty fan, or a dead rat
It’s definitely a dead rat. They creep into PC’s at dead of night looking for cheese, bite on something that electrocutes them, and then fall to the bottonm of the case to slowly rot away.
The noise you can hear is its tail rubbing on the processor fan blades or the cooling fan blades.
Either way, take the side off the case and remove the rat – all will then return to normal.