This article describes a slight "hissing" noise coming from the rear of an iMac when the computer is shut down, but still plugged into the wall outlet. If the iMac is unplugged, the hissing will go away after about 10 seconds. The hissing sound will return once the computer is plugged in again.

This noise is considered normal, and in most cases does not indicate a failure or problem with the computer. The noise is generated by a transformer in the computer's power supply, and is typically only apparent in very quiet environments, or when you have your ear right next to the back of the computer. For cases where the noise is high-pitched, or excessive, Apple recommends having the unit inspected by an authorized Apple service center.