Before you go spending money remove the PSU (usually 4 screws) to check that the fan blade is not catching on the metal guard. This happened to me once. After removing the 4 screws holding the PSU in place you will be able to slide it out - then remove the small screws on the fan guard and slightly bend it out -ie away from the fan blade. You can always check it before replacing the PSU screws. Worth a try if you are sure that this is where the noise is coming from.
iMac Pro review