Open CCC and enable ATI Overdrive.
Click the Profile tab/Profile manager and create a profile (name it standard).
It will create a file called standard.xml in (users/name/appdata/local/ATI/ACE)
Now create another profile called Fanspeed. This will create another file called Fanspeed.xml in (users/name/appdata/local/ATI/ACE)
Open the Fanspeed.xml file in Notepad. The bit were interested in is as follows.
<Feature name="FanSpeedProtocol_0">
<Property name="FanSpeedProtocolProperty" value="Percent" />
</Feature>
<Feature name="FanSpeedAlgorithm_0">
<Property name="FanSpeedAlgorithm" value="Automatic" />
</Feature>
<Feature name="FanSpeedRPMTarget_0">
<Property name="Want" value="0" />
</Feature>
<Feature name="FanSpeedPercentTarget_0">
<Property name="Want" value="0" />
Change this line <Property name="FanSpeedAlgorithm" value="Automatic" />
to be
<Property name="FanSpeedAlgorithm" value="Manual" />
Now change this line to you desired value (something between 30 and 50) 35 works well for me on an overclocked Sapphire HD 4870.
<Feature name="FanSpeedPercentTarget_0">
<Property name="Want" value="0" />
and replace the zero here
<Property name="Want" value="0" />
to something between 30 and 50
I think the changes are in increments of 5, find one that has a good balance of cooling at heavy load and is still quiet. (example 35).
Now save your file and select the changed file in CCC/Profiles and activate it.