I picked up a used Power Mac G5 2.3 Dual Core really cheap. I was told it was working but included no hard drive. It was in poor physical condition, however. Scuffs, scratches, dings and loads of caked-on dirt inside. Before I ever hit the power switch once, I knew I had to give it a good cleaning. I disassembled all parts and removed all the dirt with great care. I even had to treat some parts that were contaminated with rust (CD-ROM shield). It was obvious that the owner lived near a lot of moisture.
It only took a weekend, but I got it all back together and looking fairly decent. I connected a monitor and hit the power button. It came up! I then worked on getting a compatible SATA drive. I did not realize it at the time but a SATA rev 3 wouldn't work. I found a rev 2 and it was recognized. I installed Leopard and took care of all the updates. All seemed well for about a week.
Then one morning I was awaken by a loud clicking sound coming from the room with the G5. I confirmed it was the G5 and also observed that the power LED would flash while the clicking noise was going on. I tried to "wake up" the Mac thinking this could be some weird sleep problem but it didn't respond to mouse or keyboard input. I then pulled the plug and the noise ceased. I waited a little while and plugged it back in and both the noise and the LED flashing returned. No matter I tried, it would not power up, though.
I did some searching and came upon someone who suggested resetting the SMU as a possible solution to another's power problem. I did the reset and this seemed to stop the automatic clicking and flashing. When I tried hitting the power button, it still does not come up. I only see a momentary flash of the power LED and then a "clunk clunk" noise -- the same noise I was hearing repeatedly before.
So this is where I am. Is there anything else I can try or should I conclude that the power supply is dead? That would sure be a disappointment considering the stressed environment the machine had to put up with, now getting the proper attention it deserves only to then decide to die on me.