SMART tells you what the disk knows. Clicking in HDDs is usually caused by the head not moving fluidly over the platter.

When the firmware detects this, it tries to reset the head, usually by parking it for a moment and then unparking it. Most of the time this solves the issue.

This inherently doesn't cause data loss or corruption, just high latency when using the disk. It can lead to an increased number of high fly writes as the head is unparked and is usually a bit too high for a bit until it finds it's air cushion again.

The usual suspects in your smart data are "Recalibration Retries", "Seek Error Rate", "Head Stability" (if WD), "High-Fly Writes", "GMR Head Amplitude" and "Head Flying Hours".

All of these generally don't cause runtime issues with the drive, but a clicking sound or an increase in these numbers means the mechanical assembly of the HDD, while still in spec and good enough to operate the HDD almost normaly, is degraded.

Of course, your head may crash down on the platter the next recelibration try, the clicking causes a lot of head movement and parking, so it also causes a lot of wear. Drives which are clicking age a lot faster and in reverse, clicking is an easy indicator the drive is beginning the end of it's useful lifecycle.

Thanks for the detailed answer. I managed to identify the faulty disk, the array has been rebuilt last night.

The SMART data changed, I think it was the seek error rate that went through the roof.