Sign of the apocalypse, kie. Sign of the apocalypse.
Anyway, harddrives click when they're on their last leg, true. But I have an old 8 gig drive that clicks, whines, grinds, and everything else. But it's still running strong, and I use it for page files and temporary files. It's been three years since it was acting up, but I dont see it going any time soon.
Anyhow, you have a choice. Be on the safe side and backup everything and order a replacement ASAP, or be on the wild side and use the drive as much as you can before killing it. Or maybe you can do both :\