the first hard drive i had in my MBP failed a few weeks after i got it, and my clicking would last for about 10 seconds, click after click after click. eventually, it got so bad that it would click repeatedly and lock up my MBP, and a restart was necessary. now, i occasionally get a single click when i'm doing random web browsing and stuff, but it doesn't concern me.
yet another voice for backing up, by the way. 🙂