There are a number of failures that can cause a hard drive to click, here are just a few:
The most common reason for a hard drive to begin clicking is due to a physical head failure. This often happens as a result of the physical head coming in contact with the platter surface while in operation. This type of failure can cause slight or severe damage and in some cases make the data on the hard drive unrecoverable.
The main electronics board on the hard drive can occur damage from power spikes, often times these spikes come from faulty power supplies or from electro static discharge. While severe electronics damage will prevent a drive from spinning up, less severe damage can cause a drive to lose control of the head assembly and create clicking noise.
Hard Drives have firmware embedded in the service area of the platter service, this firmware is essentially the brain of the hard drive. It allows the drive to move the heads, platters and translate the data that is written to and read from the platter surface. In rare cases hard drives can corrupt their firmware, usually due to improper shut downs or power spikes. This corruption or “brain damage” prevents the drive from controlling the head assembly properly and generates the clicking.
Failed Enclosure or Power Supply
Sometimes the hard drive itself is in a healthy condition and the External Enclosure, Laptop, Desktop, Server, etc… are having electronics issues. These issues create a scenario where the hard drive is not provided enough power and that can cause the drive to produce the clicking noise.