I bought iMac 27" from local apple store two days ago. Everything is fine execpt that there is some weird sound coming from the computer when the screen turns darker to save energy. Once I move my mouse to activate the screen, the sound goes away. I am saying the sound is so load, but it is still noticable and a little annoying in a quite room. Does anybody have the same problem? Please let me know if you know how to fix this problem. I greatly appreciate it. Update: I search "screem+humming+dim" and turn out actually a lot of iMac owners have this problem. Is it a technical issue Apple fails to resolve in general?