I was just using it, when out of the blue it began to get pretty loud. I turned it off for a bit, and turned it back on and the fan made what sounded like a loud hissing sound and then the loud noise continued. I'm currently backing up my hard drive, and the noise seems to have quieted down a bit, but sometimes picks back up and dies down again. It sounds as if something like dust got caught in the fan and it began going a bit nuts. I honestly don't know but it's scaring me. I've had this imac since 2007 so I'm afraid it may be going soon.