First of all, depending on the exact G5 model and types of hd's installed, they're actually very, very quiet. My G5 is almost noiseless.
You have to be really cautious about an iso box for the G5, it might actually prohibit ventilation up to a point where it will overheat instead and this will in turn decrease performance (clicks and weird stuff) and fans will start spinning up.
Using external in/outtake of air can sometimes mess up the internal ventilation in the G5, causing it to actually worsen matters, I've experienced this myself.
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