Failing hard driveExplanationA clicking noise when reading or writing data from the hard disk is an early sign of a failing drive. As a precaution, you should move data from the drive as soon as possible in this case. An overheated CPU or bad power supply will cause a spontaneous reboot, not a clicking. A system notification would indicate whether there is a failed drive, as it would not allow reading or writing. A failed UPS (or failed battery in the UPS) would result in a complete loss of power to the computer if the outlet (or wall) power was lost.