Well, it could be a lot of things, but the 3 things that come to my mind right away:
Hard drive is bad. Clicking is usually the HDD. Check all the cables to make sure they are inserted correctly. Replace them if you have extras.
A fan has bad bearings. Check to make sure all the fans are spinning.
The PSU is switched to 220v. If it was, the power switch on your case will be clicking over and over at roughly .5 to 1 sec intervals. It will be like a metronome.