Do i have a failing drive ?

I've heard the screeches of death of a drive before but that sort of clicking noise is a new one for me.

This is from my unraid homeserver, noticed the noise for the first time about a week ago. It seems to be coming from one of the drives though i haven't identified which one yet. Seems to only happen when there's activity on the drives.

What could cause this sound and should i think about having a replacement on hand ?