Is it a case fan (I don't think they have case fans, but I could be wrong) or a power supply fan? Either can be replaced but replacing the power supply fan involves replacing the whole power supply, which will be expensive.
With the computer fully booted listen at the noise and then remove the jamming device and listen as the fan revs up is it that one that make all the noise.
If it is the the120mm case fan it is easy. Apple use 2pin connectors while all PC stuff use 3 pin or 12V from those big 4 pin connectors used by HDs and optical drives. A good fan from Panflo, Nexus or Adda will be much quiter. many low RPM fans running at full voltage from a 12V tap will run much quiter than the Delta fans from Apple. I prefer a steady "woosh" even if it cools to much at times than a fan that revs up and down
Getting into the PSU is tricky and the caps there can hold lethal charges. But just cutting the wires to the fan inside the PSU (not shorting them) and then attach a fan outside the PSU that is inside the case on the open end of the PSU. That fan can run of 12V. You might have to strap down the old fan so it is not spinning along rattling.
Comments
Jam something in the case fan and boot computer
With the computer fully booted listen at the noise and then remove the jamming device and listen as the fan revs up is it that one that make all the noise.
If it is the the120mm case fan it is easy. Apple use 2pin connectors while all PC stuff use 3 pin or 12V from those big 4 pin connectors used by HDs and optical drives. A good fan from Panflo, Nexus or Adda will be much quiter. many low RPM fans running at full voltage from a 12V tap will run much quiter than the Delta fans from Apple. I prefer a steady "woosh" even if it cools to much at times than a fan that revs up and down
Getting into the PSU is tricky and the caps there can hold lethal charges. But just cutting the wires to the fan inside the PSU (not shorting them) and then attach a fan outside the PSU that is inside the case on the open end of the PSU. That fan can run of 12V. You might have to strap down the old fan so it is not spinning along rattling.