It is possible (see
www.xlr8yourmac.com and Powermac G5 forum on Apple.com) that Apple placed the drive bay sensor in the wrong place and that the heat mesured by the sensor is 25 degrees less than that on the surface of the drive; given that, the sensor never reach the threshold that would incrase the fan speed. Some people, me included, had the HD in the upper drive bay damaged. The two proposed solution are to: relocate the sensor (
www.xlr8yourmac.com) so that the temp is only a few degrees less than that of the drive, resulting in spinning fans accordingly to temp variations or unplug sensor connector from the motherboard then having the fan spinnig at its medium speed versus lower.
Have you tried one or both solution? Please post here your results or questions.
Thanx
Sam