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 thing about noise levels on the HD 6990 is that they're all over the place. The fluctuation is quite high. Normally when we get a reading of noise levels it's consistent. It tends to hit that level and stay around there in 3D gaming. In the HD 6990 4GB case, though, we will see it hit around the 79dB mark for 15 seconds and come back down to 65dB.
To be honest, I think you'd be better off setting the fan manually to a noise level you can handle; probably somewhere between these two. That would prevent the fluctuation and still keep your core temps at a good level. Of course, this can all be done under Afterburner, so it's worth looking at.