I forget, does the Quad G5 have air cooling or liquid cooling?
If it's air cooling, then you may have accumulated dust on the heatsinks, which makes the fans blow harder to keep the processors cool. I suggest getting a can of compressed air and blowing all the dust out.
If it's liquid cooling, then there might be dust on the radiator, or whatever Apple used in place of a radiator. Same thing, take a can of compressed air and blow out the dust.
If there's no dust, then it's a sign that your fans might be failing. I would suggest either replacing them or getting them replaced by Apple if you're still covered by AppleCare.