Of course it is important that a power supply does its work as quietly as possible. Fortunately most power supplies are semi-passive or at the very least have a pwm fan nowadays. This means that the fan does not turn on at all or spins very slowly at low load, and only works harder when the power supply needs to supply more power.
Still there are clearly audible differences. A few power supplies remain very quiet under various loads, with measured values that do not surpass 30 dB even at 500 watts. Worth mentioning are the Be quiet! Dark PowerPro 11 850W, PurePower 10 700W and PurePower 8 CM700W, the Corsair HX750i, HX850 en HX850i, RM750i, RM750x, RM850i and RM850x as well as the Seasonic Snow Silent 750W. These power supplies produce a maximum of 25 dB at a load of 500 watts (measured at 10 cm), which means they are truly quiet.