The monitor is producing a high-pitched whine
Cause: This is often noticed in cheaper monitors, and it will become more pronounced with age.
Solution: Try adjusting the brightness and contrast controls, this may reduce the noise somewhat. You can also try ignoring the noise and let it merge with the background, like the hum from your TV. If it is bothersome, take it to repair shop; there might be some components that can be changed to reduce the noise.