Do you have a cd or floppy in it? It will autoscan every so often to see if new media has been inserted. Watch it and see if any of the lights blink when you hear the ticking sound.
Another possibility is the hard drive--the actual device inside of the tower where your data is. Some hard drives tick all the time and it's ok but if it didn't tick before and now it does--back up anything you don't want to lose and take it in for repair.