Lot's of people make a big deal about coil whine. Many think it means the card is faulty. This isn't the case. It's the physics of how coils work. It can be minimized or eliminated by the manufacturer using solid coils. Solid coils are the same as a regular coil except that they have been put in a little plastic shell and glued / epoxied / potted into it. This keeps the coil and more importantly it's core from rattling around. The physics isn't difficult to explain, but the explanation is long. Essentially what it boils down to is a coil develops a magnetic field around it when a current is passed through it. The field strength is directly proportional to the amount of current running through the coil. Since the current draw isn't constant in these coils (the draw goes up and down depending on how much power the card needs to render each scene) the magnetic field also fluctuates. This fluctuation can cause the core of the coil to vibrate which produces this noise.
Now usually coil whine is caused by wildly fluctuating framerates. The more constant the framerates, the less coil whine you'll experience. It is also more prevalent in games where you get really high framerates. In these circumstance, you can either enable Vsync to keep the frames synchronized with your monitor refresh, or use something like RTSS (RIvaTuner Statistics Server) to cap your framerate. Or you just live with it.
However all that aside, you need to determine if the noise is in fact coil whine. Since you describe that its more of a rattle, faulty fans can rattle. If you want to be sure, go into a game that you notice this sound. Make sure you can hear it. Then with the case side off, carefully and gently stop the fan on the graphics card momentarily. If you have more than one fan on the card, do each one at a time. If you stop a fan and the sound goes away, then it's not coil whine, it's a faulty fan. Usually this noise in a fan heralds the beginning of the fan failing altogether.