My 24" iMac hums no matter the brightness setting. Making it dimmer simply makes the hum louder.
When the iMac is cold, the hum oscillates in volume. When the machine warms up to normal operating temperature, it returns to a constant hum.
The hum is audible from anywhere in the room or even in the hallway. Sometimes the hum disappears for a few hours but it is nearly always present. While not loud, it isn't very white-noise-esque. It is a definite pitch that drowns out all fan noise completely.
I haven't decided if it is worth trying to get serviced. My understanding is that apple doesn't have a policy on the hum and a replacement machine is the current solution.