My Mac Pro's hard drive is making a clicking noise (not very loud, just the small crackling sound normally heard when the drive is accessing something) about every half-second. It literally sounds like the 'tick-tock' of a clock's second hand. It's been doing this for months now (maybe as much as a year) and I can't find any similar cases or relevant help by searching online. Most people say it's the hard drive failing, but again, it's not a loud click like the drive-head would make if it came into physical contact with the platter or something.
I usually work with music or a podcast going... but when it's dead quiet in the room, this noise drives me nuts. What is it/what can I do to stop it?
Here's a link to the .mp3 of the sound (pardon the background noise).
http://www.southerngamer.net/storage/harddrive_ticktock.mp3
Any help would be much appreciated.