Before you buy a new power supply, you may want to unplug the 40mm Noctua 1xNF-A4X10 to see if that helps. This fan can run up to 4500 RPM and, with my HL8, has a more noticeable higher pitch sound profile at anything above 3000 RPM. I adjusted fan curves on my custom build HL8 to keep it under 3000 unless the system was under crazy load. I also found that the cooling it provided in my setup is very minimal so unplugging for some testing should be just fine.
Ultimately, the overall power load will be the biggest determining factor on the amount of noise coming from the PSU fan. They usually run at a low RPM and ramp as you approach the max wattage. Going to a more efficient (i.e. less heat) Platinum PSU may be enough to help even at 500 watts, but also stepping up from 500W to 600W gives you more head room. All that can help the PSU need less cooling which means the ran runs slower at the same actual power usage for a quieter experience. That said, it’ll definitely get louder again upon approaching the max wattage.
Also, do you have the acrylic front? If you do, check out my post below on a modification needed to the PSU cables to support the rear Noctua fans.