Is it a flat screen or a CRT ? If it's a CRT there's a fault in the PSU, 50Hz "hum" on the line. If it's a flat screen, could be poor connections in the plugs or poss. stray pickup on the leads. try moving the leads about a bit.
Ever wonder what that "degauss" button on your monitor does besides make a buzzing noise and cause the screen to go crazy for a second? Though that's its main purpose, the degauss button has another useful feature. To understand it, you'll first need to know that the earth has natural magnetic fields. The magnetic charges from these fields can build up inside your monitor, causing a loss of color accuracy.
Degaussing scares the bad magnetism out of the monitor and fills it with good karma. If your monitor doesn't have a degauss button, fear not -- many new monitors automatically degauss themselves. If you have a flat-panel display, there is no degauss button because magnetism doesn't build up in flat screen displays.