Well, the source of the problem is in the principle of the card's power supply - it's impulse. The more current flows through the line - the higher is the amplitude of the impulse and the lower is the frequency. When the impulse frequency lowers, the sound becomes heard by a human ear (i think everyone heard a working high voltage transformer, the principle is similar). So, when you put excessive load on the GPU, it starts consuming a lot, and if the components used in its VRM are marginally sufficient - the start squeaking, especially coils and capacitors. The fact that 3DMark/furmark/games/etc. don't make the card squeak is explained by the essence of these applications: they don't load GPU or memory individually, therefore the card is almost never 100% loaded, as it waits for textures load, game and physics engine, etc. The crysis loading screen or 3DMark Vantage's loading screen, for example, make the cards squeak because on each first load for each VGA and each driver version they compile the shaders from source, using the GPU.
In fact there's nothing you can do and nothing to worry about, just ensure your videocard is properly cooled. Though, if you have some soldering skills and are not afraid to lose warranty, you could try to replace the capacitors in VRM with bigger ones, that should mute the sound.
