After installing OSX 10.7 Lion your CPU might go to 100% and not going down. As a result the fan is turning to a maximum speed.
There are two possibilities what’s happening.
1. Lion is indexing all your files. This might take some time. While indexing your CPU will be used to 100%. This is normal and no need to worry. You can see if your System is indexing if you check the spotlight icon in the top right corner. If the magnifier has a small dot in the center your system is indexing.
2. If there is no indexing in progress go to activity monitor and see what process is causing the problem. One of the process can be AGAdminService running as root. This process can be part of Citrix Online Plugin. If you have Citrix Plugin installed you can try to uninstall it. If it’s not working and the process still on 100% load, you have to execute the following command in a terminal session.
sudo launchctl unload -w /Library/LaunchDaemons/com.citrix.agadminservice.plist
You’ll have to enter your root password. If you execute the command the Service should disappear and shouldn’t appear at the next logon.
Thanks for this solution to mahbool in the following thread in the Apple support forums.
Link to Apple Discussions