After installing 10.4.3, I was dismayed to find that it had sent my G5 (a first-generation Dual 2.0GHz model) into the land of eternally cycling fans. Every few minutes, for seemingly no reason at all, the fan noise would ramp to near-max for a few seconds, then spool back down to idle. Watching with top, there was seemingly no cause for this activity -- no process was sucking up the CPUs, and everything looked completely normal. Needless to say, this was extremely irritating -- if you've ever heard a G5's fans at maximum speed, you know how loud they can be. It was so annoying I was contemplating downgrading to 10.4.2, just to get things back to a quieter noise level.
I tried the basics -- a simple restart, a SMU reset, and a PRAM/NVRAM reset. None did the trick. Then a friend pointed me to another possible solution he found at MacFixIt, involving (of all things) the Energy Saver preferences.
Open the Energy Saver System Preferences panel, and click the Options tab. Click the Processor Performance pop-up menu, and set it to either Highest or Lowest -- anything other than Automatic. Close the panel, and you're done. Not wanting to give up any performance, I set mine to Highest. I made this change on Friday afternoon, and since then, the fans have returned to their pre-upgrade behavior. They now only get slightly louder when I do something CPU intensive, and are at the same nice-and-quiet level the rest of the time.
Also see this hint for a possible solution if you have other noises in your G5 that you can't seem to explain or silence...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051106063023177