i have a 27" i7 also and I think it is one of the fans. its on a wooden desk and I havent tried putting a towel or mousepad underneath it or anything. it's sort of an oscillating, low pitch noise, that will get louder and softer, continually. i'm going to hold out and live with it for a while, once the i7's are more widely available, and I have the time to deal with replacing it if necessary