It's a fan bearing going.
Normally two choice, CPU or power supply (you may have extra case fans, in which case it could also be those)
Although probably not best practice as such, if you can't easily hear which one is making a noise just stop the fan by putting your finger on the centre of the fan (not in the blades!) and see which one stops the noise. if it's the power supply one then strictly speaking you need a new power supply (it's possible to change the fan, but you're not really meant to, if you do attempt to change a PSU fan take care not to touch the caps as they can be live for hours after switch off) if it's CPU you normally need a whole new cooler.