If it is loud, it might need a thermal calibration. As far as I know, only an Apple tech can do this for you. Or it might be a problem. Unless your fans are particularly clogged, changing them out probably won't help. A widget like iStat Nano will tell you fan speed. Perhaps others here could report theirs for you to compare. Most G5s are pretty damn quiet when running properly, but they are seriously loud when they aren't.