The click you're hearing is most likely related to the read head failing to track when it's writing/reading something on the platter.
It's essentially trying, and failing to do a task.
(The sound is the head pretty much slipping back from the platter slightly).
You will need to backup the data on to a different drive entirely as soon as possible, it doesn't necessarily need to be another mechanical hard drive. Anything from a memory stick to a CD should do.
What will happen now is that it will gradually get worse and worse to the point where it fails frequently and creates a whoooole lotta' corruption on the platter. (In other words, bad sectors).
You might get lucky and be able to claim another one if it's still in its warranty period, quite a few brands out there have some nice warranties to go with their drives.