With our recently acquired G4 MDD, the Windtunnel does stand up to it’s name! We replaced the fans in the PSU with 2x Evercool EC6015M12CA 80mm fans. They push out 16cfm, which is half of the original fans but the same amount of air comes out so no worries here!
The next problem was the Case fan, which is a huge 120mm fan that we replaced with a cool-looking Cooler Master Sickleflow 120mm fan. The MDD is now a million times quieter and is much more tolerable. The G5 is still quieter than these fan upgrades unfortunately!
To replace the fans do the power supply first. iFixit has a great disassembly guide to take the PSU out. After that is out (you do not need to unplug all the wires to the motherboard) remove the screws on the housing, then remove the old fans. Snip the black/red wires from the old fans and re-wire those up to your new fan. Screw in the new fans and you are good to go to put the PSU back together!
The case fan is much easier to replace. Remove the Optical Drive Cage then do Step 17/18 to remove the old fan by pulling the old fan up. Reinstall the new fan. You can either try wiring the fan to the motherboard or use a Molex to 3-pin power. Instant quietness!
Good luck with your MDD quieting!