AFAIK the only fan in the PCI G4 (you men the 350Mhz Yikes model right?) is in the power supply and will always stay on. It shouldn't be very loud though. I have one of those machines at work and the fan is nearly silent. You might just need to blow the dust and other cruft out of the power supply with some canned air. I have heard of people replacing the fans in power supplies with ball bearing fans to silence them. You might want to try that if it really bothers you and you are comfortable disecting the power supply.

-Mike