Clicking sounds are usually the first indicator of a problem with an external hard disk. Frequently, however, the cause is not a physically broken drive.
There are two main sounds that can be described as clicking: soft clicking, and hard clicking.
Soft clicking can be the normal sound of the drive working. If the drive is functional, this is normal. Drives do not typically give an indication of any problems prior to failure, so it does not mean it is about to fail if the drive is making a clicking sound and still functioning. If the drive is non-functional, there are two possible causes:
Hard clicking is a very noticeable sound, and is akin to hearing metal-on-metal impacts. It is loud enough to interrupt conversation, and can definitely not be ignored. Sometimes this sound is accompanied by an alarm or beeping. This behavior usually indicative of a physical failure. If nothing traumatic happened to the drive prior to this starting, consider it to be soft clicking, and troubleshoot the problem as suggested above.