Replacing power supply fan with a quieter fan.

[jarsoffart]

I just received a 92mm Adda fan (model:AD0912HS-A70GL). I have a Turbolink 420w PSU. I want to take the bottom 80mm fan and replace it with the 92mm fan, but I don't want to rout wires out of the PSU and connect it to the external Molex connectors of the PSU, I want to connect it to the internal PSU 2-pin plug. I plan on soldering the 2-pin connector of the old fan onto the new Adda fan. The 92mm fan though outputs 33.7 dBa of sound, so I want to undervolt it. Which resistors would I use to get it to around 20 dBa. Currently the fan's amperage is at 0.25A.