While much has been written about the placement of the drive bay temperature sensor on the first generation G5 Power Mac, I was unable to find information about what to do in case of sensor failure. In my case, the drive bay sensor reported temperatures oscillating between 77 degrees and 255 degrees in less than one second!
After trying a PRAM reset, a PMU reset (my model does not have a SMU), resetting NVRAM in Open Firmware, etc. to no effect, I finally disconnected the drive bay sensor from the motherboard. I expected all nine fans to rev up to full speed, but instead my fan noise problems disappeared! Furthermore, the drive bay fan, as well as the rest of the G5's fans, still spins up in response to the remaining, functioning sensors.
As usual, all disclaimers apply -- doing this can ruin your machine, and will certainly void your warranty, and should only be done as a last resort.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061002184713980