Could be a run-down bearing inside the fan. Did it come with the case or is it an aftermarket one?
Worst case scenario, it'd just stop working. Though, it's a fan, easy to replace.
However, to check the unit and the PWM-cables aswell. Make sure there's no shortages or any other damage on unit and cable.
You could also use some kind of software to see how many volts is beeing fed into the fan. Make sure it isn't overvolted.
SpeedFan and HWMonitor are solid softwares.
Good Luck!
Axel.