I can tell you what it might well be.
Since I put my WD Scorpio 320GB hard drive into my macbook, I have been noticing little 'tick' sounds while surfing the net.
They were definitely not loud enough to be indicating an imminent failure (and unlikely since it was a brand new drive), but they were loud enough to be annoying when I am in a silent room. And they were happening every few seconds, and really driving me nuts.
After a long net search, I have found the problem.
Mac OS X parks the 'heads' of your HD as much as it can to protect the hard drive should the laptop suddenly move, and to conserve power. Unfortunately, it happens so much that it causes 2 problems:
1) Each tick is known as a 'load cycle', and too many load cycles causes premature HD failure
2) On some drives, the tick is loud enough to be highly annoying in a quiet room. But, Apple's hitachi drive didn't seem to be loud enough that I noticed it at all. I only noticed it when I put the WD drive in.
The 'solution' to this ticking is not exactly great. It involves downloading this:
1)
http://mckinlay.net.nz/hdapm/
2) Dragging the UNIX executable file to /usr/local/bin (you'll have to create the 'bin' folder)
3) Running that file
4) Opening terminal and typing - "hdapm disk0 max" (without quotes)
Now, this stops the drive from EVER parking the heads, so you will never hear the ticking again.
BUT, it causes two more problems:
1) The head parking does actually protect the drive from movement damage, so you are increasing the chance of failure
2) It messes with the automatic sleep function which means you have to close the laptop to get it to sleep, or go to apple > sleep. As long as the laptop is left open, the hard drive will keep spinning and spinning forever!
AND, you have to open terminal and type 'hdapm disk0 max' every time you reboot and sometimes when you switch to battery power.
Hmmm, 'it just works' ?
