If you're still running 10.7.4 try updating.
If the Focusrite driver let you change bitrate try that also.
This is typically caused by a low buffer size. Please raise the buffer size in your recording software, and the crackling should go away.
If you're still having issues, contact our tech support team here for more help: http://www.focusrite.com/answerbase/en/contact.php
David Tichauer // Focusrite Technical Support