Audio Hiss on 27" Cinema Display

Hey guys,

Yesterday, my 27" Cinema Display started making a low-volume audio hiss sound whenever a sound is being played.

For example: the monitor is completely silent when I first boot, but if I go to change the volume, the hiss sound starts, the volume-changing noise plays, and then the hiss continues for 30 seconds or so, then stops until another noise is played.

If a video or something is being watched, the hiss is in the background at all times.

Any ideas what I should do about this? It's really annoying.

Thanks