If your PSU is nearly at the end of its life, you will notice it most probably during times when the activity of the PC is high, the PC will most likely reboot.
Are you sure it isn't your harddisk which makes that clicking sound? I have heard stories about harddisk clicking and then crashing after a while, maybe you want to check the health status of your HD with tools from this site for example
http://www.ultimatebootcd.com/. If it is your HD, better start making backups before it is too late