This is caused by a defective capacitor in the power supply board. A very detailed answer is given by stiv0. His solution is for a "no power" or "black screen" condition, but this also fixed my noise on power up. I had noise that went away, but eventually it stayed on all the time. His explanation is correct except in describing how to get the back cover off. He says to take the 3 screws off the back of the black plastic cover on the back and then "pry" up the plastic. Actually if you slide the plastic down about 3/8 inch the cover will lift out easily. I fixed mine today in about an hour.

Hope this helps.

Trent