The general Mac OS X help suggests this may be due to "interference from a nearby power line, a fluorescent light, a radio, a microwave oven, another computer, or another electrical device" and that relocating such devices or the computer/display may resolve this. If it persists despite making changes to the nearby environment you should probably bring the display in for diagnosis. Generally the Apple Store in your area will offer complimentary evaluation of the device in question but whether the repair (if one is required) is billable or not is primarily going to be determined by your warranty status.