Quoted from andre060:
Seems likely you do have power to the backbox then - I'd move on to checking the 5V and seeing if the CPU is running. Start here: https://www.pinwiki.com/wiki/index.php?title=Williams_WPC#Power-On_LEDs_and_Sound_Tones_.28Bongs.29
Not necessarily....I have seen similar before where it was the power cord at fault. Check both ends of that power cord very thoroughly. A lose connection can cause the clicking. Also check the main power fuse holder. The click can be a relay momentarily getting enough juice to energize, or arcing in a connection. Start simple first.