I recently received a 20" iMac from a friend who said it was broken and said if I can fix it, I can have it. Its model number is A1224. When I go to turn it on it makes a horrendous noise, almost like a piece of paper getting hit by a fan blade, very rapidly in the lower right hand corner. It then stops for 2 seconds, and does it again. All of this happens while the screen is blank. Any idea what this might be? I know I got it for free but I am trying to fix it so I can give it to a family member.