Ok I got my mac last summer before they redesigned the iMac and ever since I've had it when I turn it on either there's a LOUD noise out of no where or there's a crackling noise that goes on for 10 seconds. Also, when I open up iMovie and turn on the built in iSight a loud noise comes. Now, I called mac months ago and they said I should go to my nearest Apple Store(for me it's about 1,000 miles away)and get it checked out. I think it's a wiring problem in the speakers.