While in Windows 7 the Radeon 6870′s GPU fan is almost inaudible under idle conditions, the fan is clearly audible in OS X Lion. In OS X the GPU fan is permanently changing its speed which creates quite some bothersome noise. If you can’t live with that noise, here’s a tip for the not-so-faint-of-heart about how to modify the fan control curve of your Radeon graphics card. This involves flashing your graphics card’s BIOS using an optimized temperature/fan-speed map. Ain’t that cool? :-)
Follow this guide on insanelymac.com. But instead of using the look-up table, you just select “set all fan settings to recommended values”. Flash the BIOS and you’re done. No more fan noise under idle conditions in OS X Lion.