Thermaltake Toughpower 500W Modular PSU Review
 

The bottom of the PSU has a 140mm fan on it, this is big. So
the fan can push more air while spinning slower (compared to smaller
fans). This means that it is very quiet. And with the included silicon
dampener, noise will be at a minimum.

http://www.xtremecomputing.co.uk/review.php?id=376