There is the off chance that what you are hearing is normal and you've just started noticing it for some reason. However, clicking usually indicates that a drive is nearing its end of life. This can happen even with new drives due to a manufacturing defect, damage caused by physical jarring or environmental conditions, or electronic component failure due to a power surge. Because there have been no unrecoverable read/write errors yet, the SMART data may be OK even when there is an electronic or mechanical problem.
It's time to back up all of your important data and get a new drive. The easiest way to migrate to the new drive is to just clone the old one. However, with a failing drive you can never know whether or not it may crash in the middle of a cloning operation. I'd suggest saving your data to an external drive (if it's really important data, make two backups and verify them against each other), then a clean install of the operating system and applications to a new drive. If your old drive is still under warranty you might see about getting a replacement but the drive company isn't going to save your data for you so you will need to backup it anyway.
According to a now-deleted WD web link:
Issue: A Western Digital hard drive makes a repeated clicking noise.
Cause: There can be several reasons why a WD drive may be making a clicking noise:
- The drive is not getting enough power to fully spin up
- The drive has failed
- Occasional clicks during data access
- Hard clicks during a head park operation (shutdown or sleep mode)
- The data cable is faulty, or incompatible