I know this may be WAY late in the game but I recently acquired an HD 6870 to rebuild my first computer.
I believe the fan is thermally controlled. It will ramp to 100% at start up then slow down once out of BIOS. Sometimes those issues are due to drivers sometimes it can be a power delivery issue and your power supply isn't enough. I had a nVidia (yuck) 9800GT that would throw a fit when you gave it an intense workload but then I got a 650 watt power supply and that made it happy.
My questions for you are what version of Windows and what driver are you using?