Under normal load and generally "acceptable" FPS (I guess as high as 300), I noticed my Radeon HD 5570 and 7850 have no real issues. Whenever games and benchmarks tend to go into 1000, 2000, or even 3000 FPS, I notice both cards tend to make some noise, which from reading, I believe is coil whine.
Is this normal behavior for cards? As I mentioned, normal gaming and complex benchmarks don't cause any coil whine at all, it's only games with uncapped loading screens (GTA IV) and Triangle of Death (Kombuster) I notice that both cards will start producing coil whining, and usually the FPS is some insane amount too.
The sound your hearing is likely coming from your power supply, not your video cards.