Graphics card fan too loud
Loudness can have several issues. Rik already mentioned one. If the cooler is dusted up, the card gets hotter and thus the fan runs faster, so cleaning the cooler can help lots.
The fan can also get louder if the ball bearings are giving out. That usually causes not just an increase in loudness, but a rather distinct sound that goes beyond just the fan being loud.
You can "oil" a fan if it has a sticker in the center of the fan blade that covers access to the ballbarings. Check out this link to see how it's done: http://www.dansdata.com/fanmaint.htm
Also, you can lower the speed of the fan by using software such as Nvidia Inspector. Many manufacturers also supply tools for this. Usually, you can't go below a certain threshold. You can combine this with undervolting the GPU for the card's 2D-mode which will result in creating lower temperatures allowing you to speed down the fan even more. This makes my GTX260 with dual fans basically silent. It should be a given to always use a temperature monitoring tool for this procedure and read up extensively on how to do that before attempting it!
To lower the fan speed for ATI cards open CCC and enable overdrive. Create a new profile. You can find the newly created profile here:
XP: C:\Documents and Settings\USERNAME\Local Settings\Application Data\ATI \ACE\Profilename.XML.
Vista/7: C:\Users\USERNAME\AppData\Local\ATI\ACE\Profilename.XML
Open the file in a text editor.
<Feature name="FanSpeedAlgorithm_0">
<Property name="FanSpeedAlgorithm" value="Manual" />
</Feature>
<Feature name="FanSpeedRPMTarget_0">
<Property name="Want" value="0" />
</Feature>
<Feature name="FanSpeedPercentTarget_0">
<Property name="Want" value="60" />
Change the value for FanSpeedAlgorith to manual. Then you can set FanSpeedPercentTarget to provide a new target speed that overrides the card's BIOS setting. Save the file and re-load the profile to activate it. You can also copy the above example. It will run the fan at 60% and thus slower and less noisy. This has been confirmed as a fix by Radeon 4850 users on the following forum:
http://www.tweakpc.de/forum/tutorials/57531-ati-radeon-hd-4850-fan-speed.html