Enermax Liberty 500W Modular PSU
 

I tried to listen to the PSU fan kicking in, but the only way I could tell it was running was by looking at the motherboard

Monitor status and putting my hand over it. It does run silent, and quite often would shut off, even under load, as the PSU was running cool enough. It is a testament to the efficiency of the PSU, as really any heat generated by a PSU is the power that is being wasted during conversion from AC to DC. This is a really good design for a silent PSU that doesn’t go over the top with heatsinks sticking out everywhere.

http://www.xtremecomputing.co.uk/review.php?id=214