Ive had this problem for a long time.. I have an iMac g5 2.0 ghz.. sometimes i hear like this buzzing vibration noise, the type u hear like when theres high voltage wiring.. It all started happening after i got my computer back from compusa when it was being fixed.. I finally figured out that it was coming from my back panel of the imac..the bottom center.. When i like clamp the case together with my fingers the vibration stops.. I can feel that the middle of the case isnt sitting fully on the front of the computer.. When i tighten the screws the noise goes away for a while. What can i do? The noise doesnt happen all the time, just sometimes..