It's loud even right after booting up. Ambient temp in my room is about 23C. I tested it also with just with a motherboard and P4 2,4Ghz plugged in to minimize the power draw. It was as noisy as with everything plugged in(Audigy 2, Sapphire 9700 Radeon, 3Com NIC, Seagate Barracuda IV 80Gb). The noise increase when things heat up around 32-33C is barely noticiable. I thought first that the Radeon fan would be more noisier than everything else in my system but I was wrong, it's the Truepower.
I have four Papst NGL fans, one is blowing air out and the other sucking it in @ 12 volts while the other two are on Fan Only connectors. Papst NGML is on the P4.
1) Ambient temp inside your case will matter A LOT. I just put the thing in my K8 box which generates hughe heat (35c inside case) and thing is as loud as any power supply is. It is housed in el chipo case ($50?) which doesn't give me much room for air flow. Getting my dremel readied up for coming weekend. In my P4 box where inside case temp is about 27c, it's sound is minimal (other than the coil buzzer)
2) Right when you boot up, it spools up to full speed to make sure the fan is running.
The ultimate in silence (well not using water that is) seems to be finding the critical temperature where all components are happy. Where optimal air flow can generate huge improvment.
3) If you have unsued open vents in case (no fan on it), check if it is sucking air in or out... I found out that by making it so there is no negative or positive airflow, it generates much more constant flow of air that helps keeping the temperature down on CPU & PSU as much as 3-5c.