It sounds like a Filter Capacitor/s in your Monitor's Power Supply, has gone bad causing 50/60Hz Hum. But, although highly unlikey, the problem could also be your Video Card. Have you tried a known good monitor or have you tried the suspect Monitor on another PC? What is the Make/Model of your monitor?

Most "bleeding video" problems are associated with bad caps on the CRT neck board. Start replacing them systematically; first, the 3 or 6 closest to the Red, Grn, & Blu pins of the CRT. Next, the caps around the voltage source coming into the board. Next, others that may be connected to the video output of transistors or IC's. Or, if there aren't that many caps, replace them all at the same time. Another possibility is poor voltage from the power supply. Check the filtration of the DC outputs.