Purchased an iMac off Amazon back in April of 2010, running Snow Leopard. Recently, the machine occasionally will start whirring/clicking.. best I can explain is a 1 second whirr, followed by a click, repeat. I can't consistently get this to happen. The machine typically has to be force powered off, and then it comes up on a reboot. Two reboots ago, it didn't come back up, it just kept going with the whirr/click. Force power off/on again, and I'm here typing this. All my data is backed up to Time Capsule, no worries there. Yesterday, I took the machine into my local Apple Store. They ran some diagnostics on it (Disk Verifier, then I believe an fsck from the command line).. found some minor errors regarding number of files/volumes or something if I recall right, and fixed. After that, I was unable to reproduce the whirr/click, so I figured ok, maybe we're fixed, took the machine home. Was fine for about 18 hours or so, when I just got the dreaded whirr/click again. What's my next step? Back to the Apple store for a HD replacement? Could it be any other part of the computer? If the HD is dying, I'm surprised I'm able to boot back up the majority of the time and continue working for hours. Thanks!