This is a problem I had a year ago, and just got curious about as it is VERY confusing to me.

I downloaded lubuntu lucid and it worked fine and the monitor displayed correctly, but after about an hour the (crt) monitor began screeching loudly. I plugged another monitor in, the same model of Dell, and it worked fine, after about an hour, though, it began to screech loudly, still showing a perfect image. I did a lot more experimenting and found that no matter what hertz and resolution I set it at, how many times I reset it, and what type of graphics card or crt monitor I used, the monitor would begin to scream at me after an hour. When the same computer boots any os other than lubuntu, it doesn't do it, and when I moved the lubuntu hard drive to another computer, it didn't scream after an hour. It seems to be an incompatibility between the software and the motherboard? But then what signal could possibly be being sent to the monitor that makes it wait an hour, then scream at me. Also, turning the monitor off and on again reset the hour.