I never had a problem until a few days ago, I think mine is ready to go.
I hear a low frequency, but high pitch sound coming from my PSU when the power cord is connected, all this while my Xbox is turned off.
It sounds very similar to the low frequency sound hear from your television when it's on but you have it on one of the empty video channels. You know the ones with the black screen and no sound.
It's pretty strange, the sounds kicks in a few minutes after I turn my Xbox off.
I unplug the power cord from the xbox and it goes away, obviously.
At this point I think my PSU is screwed but I'll get the replace power cord just to see if it makes a difference....
I have a 1.2-1.5 PSU having the same low level high pitch sound... neither the old or new cords solve this problem. Nothing seems burnt on the PSU...