Just over a month ago my 2008 Mac Pro started to make a clicking sound for a few seconds during the boot process and inside of either OS X or Windows when playing HD movies, HD Flash movies, when rendering in 3DS Max, and most loudest when playing 3D games. It will occasionally do it when scrolling through a rich content web page, but to a slightly lesser extent. I have been able to individually check each hard drive and cooler fan bar the one inside of the PSU as its not accessible but have checked to cooler close to the dvdrw and know its not that one either. I clean the system out twice a month with compressed air, and the sound is not there when viewing plain text still image web content or when viewing SD movies, or just playing back music in iTunes. Any suggestions would be greatly appreciated.