the PowerMac G5 had been quieter then the Powermac G4 windtunnels
and my PowerMac G5 2.3 dual is not loud at all , on setting automatic most of the time i cant hear him at all apart from a slight wirrrr noise but far from loud , i had louder pc's in the past , after all its a desktop
so which of the fans is the culprit as thats a indication which part has a problem , install istat menu's 2 to watch temperatures and fan speeds
but a general advice if possible never ever let the room temperature rise above 18 degree celsius (64.5 degree Fahrenheit ), the PowerMac g5's are more of the nordic type and thats where hey feels most comfortable 😀