I have an ATI Mobility Radeon HD 5870. Under Windows the GPU fan is inaudible, but under linux (radeon OSS drivers) the fan stays at 100% (very
loud). I have already done this:
# echo profile > /sys/class/drm/card0/device/power_method
# echo low > /sys/class/drm/card0/device/power_profile
(source: ATI - ArchWiki
Now it just fluctuates between medium and high speed. Is there anything else I can do to make the fan slow down (besides use proprietary drivers)?