Yeah, I think I do know what you mean. I have the same thing myself, only under heavy load. The best explanation I've come up with is coil whine, which is a weird electric phenomenon in which the oscillations in the current in parts that use a lot of power (GPUs and PSUs especially) make them vibrate, and the vibration produces that sound. http://en.wikipedia.org/wiki/Coil_noise
The artifact thing is a completely different issue. Coil whine is usually harmless. Run a stress test (MSI Afterburner and EVGA Precision both include them) and see if it logs any artifacts. I think you'll probably see some if you can get them in games. That's just for information, anyway: you'll still probably need to clock down a few mhz to get rid of the JC2 artifacts. Take it down 10mhz at a time and see if you can reproduce the issue.