My 480W U15 PSU has been working for months with an S-FLEX 1600rpm at 5-6V without problems. I have a quite power hungry system (160W+ idle) in an Antec P182
I have the same PSU and have replaced the Fan with an S-FLEX 1600rpm. At first I connected the fan to the PSU's fan controller which I measured to provide ~4.8V when my system was idling. Because it was in the middle of the summer and the (little) air coming out of the PSU was rather hot, I connected the fan to an external fan controller and fed it with ~6V (lowest setting on AKASA fan controller). Note that I did that only because I felt the air rather warm for my liking, although the PSU had already worked fine for several days like this.
Now that the winter has finally kicked in I'm thinking about connecting the fan again to the PSU's controller to lower the speed.
My system consists of a MASSIVELY overvolted Athlon XP (~2.1vCORE) running at 2300Mhz on an Abit NF7-S, with a GF4 Ti 4600 VGA, TV tuner, 3 optical drives, 1 raptor 74Gb and one plain 7200rpm storage drive. The PSU is in the bottom chamber of a P182, along with the 2 hard drives, with the Fan removed. I always have the door of the case open and have also moded the front grill to make it more open (I have kept the filter on).
I don't have the idea that my Noctua fan is controlled at all. It is always very audible. I have another one in my system which runs at 5 volt and that one is very quiet. I'll try placing the PSU at the bottom of the case (cooler master stacker). I think this should be possible. Then I'll let the fan run at 7 Volt (fixed).
Anyone alse have suggestions?