The 80 PLUS Platinum efficiency and Zero RPM fan mode mean that at low to medium loads, the fan doesn't spin at all. And when you push the HXi hard, the thermally controlled fan only spins as much as needed.
But that's not all. It's quieter than most PSUs even when the fan isn't spinning. The HXi is built with capacitors and other internal components specifically selected to reduce coil whine and other electronic noises.