It's been going on for an hour and I can't figure out why. Searched and it appears some have had fan issues, I can't tell if it's a fan or a drive. It's on the right side of my keyboard close to the power button, at least that's where the sound appears to be coming from. I ran the Apple Hardware test, it errored out the first couple times, but I'm guessing it's because I didn't have it plugged up to the wall. Once i plugged it in it ran fine with no bad results. I downloaded and ran iStat and it looks like my fan on the right is running about 2100 RPM and the left is 1900 RPM...so I'm guessing it's not that? I repaired permissions, restarted, shut down, etc. This is a constant hissing/grinding/whirring sound that is driving me nuts!! Any suggestions? It seems I'm going to have to take it in to the Apple store. But any ideas are very welcome.