After returning 4 21.5" imacs with yellow screens and hard drive noise, I decided to try a 27" and finally a beautiful screen with no problems, no hard drive noise. I was very happy...untill I started hearing a humming noise that sounds like it is coming from the power supply. The noise stops when I put it in sleep mode. I tried a bunch of things to determine if it was the fans or hard drive and that was not it. I am pretty sure it is the PSU humming all the time. This is the first imac with a perfect screen and now it is humming. I am so frustrated. What should I do? Have the power supply replaced or return it for another one and take the chance of getting one with a yellow screen? I am still within my 14 days of original purchase date.