If your G5 case makes a rattle or a vibration, try my solution.
I recently installed some new RAM into my G5, and decided to remove the thin plastic pieces (they look kind of like tape) on the door that Apple puts there when manufacturing the computer.
If you have removed these pieces, and have a rattle or vibration from the case, it may be because those pieces were originally dampening any vibration noise coming from the aluminum case.
To get back that nice silence, simply take 3 strips of packing tape and apply them as seen in the following picture.
