I purchased my iMac 2.8ghz 24" in July. It's been running fine but just recently, it's been making this noise that comes from the back of the iMac (middle top). If I put my iMac to sleep, it goes away and I can hear it power down. It does not sound like a fan and I checked my istat and fan speeds are the same as usual. I called Apple Support and they too said it wasn't a fan issue but they weren't 100% sure if it was the harddrive. Could this be my hard drive making noise? Do I need to make an immediate backup to make sure my data is secure? The problem is, it comes and goes, so if I bring it into the Genius bar, I may not be able to replicate the issue. It doesn't sound good and I know it shouldn't be making the noise.