is the PSU fan located on the bottom of that unit? that's what it looks like in the pic. Is the mounting board solid beneath that, or did you cut out a hole to allow the fan to ventilate? Could be a simple problem, much easier resolved than replacing the PSU fan...
There is no fan on the underside of that power supply. It just has fans on either end and I think. There must be something inside of it like a 3rd internal fan or something because it definitely has most of its noise coming from the power supply. Neither of the end fans are causing the sound.