Are you sure its the ATI card fan and not the G5s fans? The fan on my x800 burnt out, and generated lots of heat causing the G5 fans to spin faster and louder than usual. Eventually my x800 overheated and got damaged, I sent it in to ATI and they replaced it, luckily I held on to my old 9600 to put in the G5 while it was getting replaced.
Anyway, I would make sure that that it is in fact the ATI fans spinning, if it is, then maybe they are dusty, that fan spins fast and dust may off ballance it causing it to sound louder than usual. Really though, after my x800 melted, I am happy to hear the fan spinning😀