Are you sure it's not your computer making the noise? The G5s can make a sort of screeching noise under certain circumstances (as can some graphics cards), and since it's happening with both of your monitors, that seems a lot more likely than it being the screen.
Why I say that (sorry, I'm a geek, I get carried away):
CRTs make all kinds of unplesant high-pitched whining noises due to the electron gun's scanrate and other electronics, and those of us with sensitive hearing can find the noises very annoying unless the refresh rate is high enough to put it above our hearing range.
That said, I've never in my life heard of an LCD doing the same thing, and I can't figure out how one would other than funky electronics in the processing section, in which case what's on the screen would, I suppose, make a difference.
I say this because in a TV, you have an electron gun scanning back and forth about 15K times per second, a frequency some people can hear and is very annoying if you can. CRT monitors have higher (and variable) refresh rates and numbers of horizontal lines, so the gun is usually much higher pitched and well above human hearing.
LCDs, on the other hand, turn on each row individually and send an electronic pulse to each pixel; there's nothing moving, and even if there were even small, old LCDs would theoretically whine at something like 30KHz (768 rows X 45Hz refresh rate), which is way above what any human can normally hear. The only other thing that might make noise is the backlight, but that shouldn't be audible either, and certainly wouldn't change based on what was on the screen.
I'd be interested in hearing somebody more knowledgeble about this come up with a theory.
