Pulling out the TES 1350A Sound Level Meter we find ourselves quickly yelling into the top of it to see how loud we can be.
After five minutes of that we get a bit more serious and place the device two CM away from the fan on the card to find the maximum noise level of the card when idle (2D mode) and in load (3D mode).
The card actually manages to get quite noisy at times under full load. What's interesting, though, is that while the noise is indeed quite loud, it's more of a "hum" vs. a "whine." That will probably not make a lot of sense to people, but if you've dealt with smaller fans that offer a more high pitch noise, you probably do get what I'm getting at.
The "hum" the GTX 580 Lightning has is much easier on the ears vs. the noise a HD 6990 makes. The fact the HD 6990 also fluctuates a lot is annoying as well, as your ears never adjust.