My 2004 G5 has made a "click" since day 1. When it powers on or wakes from sleep, there is a little click then the fans rev up. When it shuts down or goes to sleep, there will be a similar click then the fans stop right away, followed by a kinda "power down" sound about 1 or 2 seconds later.
Seems normal to me. Just sounds like a switch or something actually. Then again, the wiring where I am is not grounded. By I have been places where it was grounded and it made the same sound.