That’s the noise my iMac started making yesterday, just before freezing completely and subsequently refusing to boot. It was coming from the hard drive. I took it into GBM for warranty repair this afternoon. I’m fairly sure they’ll have to replace the drive.
Fortunately I’ve not lost too much stuff because I had recent-ish backups, but there are still some photos and personal stuff on there I won’t get back. Ho hum.
UPDATE: It’s back, and it was indeed the hard drive. Lots of stuff to reinstall!

Hiya, Noticed the incoming link from Technorati. Nice site
Sorry we couldn’t rescue your data, the HD was just too badly damaged I’m afraid. If you have any further problems or questions, please give us a call.
Haha, I just got your picture on Sketch Swap, and I couldnt help but come look at the website. Yeah it is like drawing with a brick.
Wow. That was months since I went on Sketch Swap. I’d completely forgotten about it!