"Head clicking" is an informal expression referring to a hard drive that is making strange noises or making unusual noises. These noises could indicate a hard drive malfunction and could be a sign that something is wrong.
In the specific case of a Western Digital 500GB hard drive banging its head, this could indicate an internal mechanical problem. Hard disks are made up of magnetic disks that spin at high speed and read/write heads that move over these disks to access stored data.
When a hard drive is head clicking, it could be that the read/write heads are damaged or out of alignment. This can be caused by a variety of reasons, such as physical shocks, a hard drive crashing, or wear and tear over time.
Knocking noises can be characterized as a repetitive "click" or a constant "ticking" sound. These noises are produced when the read/write heads try to access areas of the disk that are damaged or inaccessible. The hard drive may repeatedly try to move the heads to reach the problem areas, resulting in knocking noises.
...
https://www.youtube.com/watch?v=NPq36ZBOKWA