I would suggest the Seasonic S12 Energy Plus 650. They make real quiet PSUs with a single 120mm fan.
Seasonic makes the Silencer 610 for PC Power & Cooling, but it uses a 80mm fan instead of a 120mm fan. Therefore, the fan must spin louder to move more are than a 120mm fan. That will be my next PSU.
Or the Corsair HX620. Also made by Seasonic and very quiet.