My two PowerMacs sound a bit like helicopters, so I decided to try to replace the case fan. The existing fans are 120mm x 120mm 12V DC, so I spec'd out a modern quiet PC fan of equivalent size and similar airflow ratings:
Both NewEgg and Amazon carry them. The fan is a standard 4-pin PC fan (pwm for speed control, speed sense). As neat as it might be to wire up the speed control, the fan runs more than quiet enough for me -- just hook up 12V+ DC to the yellow pin, and ground to the black:
Not strictly hardware hacking, and not at all a 68k mac, but whatever: quiet fans!