"Clicking" is perfectly normal, it's a question of degrees. All HD's make noise when accessing/writing data. The "click of death" that many people refer to is actually a much louder sound, more of a "clack", and if usually accompanied by a brief system freeze. That said, if the drive previously made no such noises, but now does, it is certainly suspect.

There are many fine HD diagnostic tools available, mostly from the drive makers themselves. If a drive passes these tests, it can be considered healthy. If the drive came with a disc, the tools may be on that disc.