Martin over at XS did some testing on the optimal shroud size... and you don't want more than about 30mm or it actually reduces the effectiveness of the fan. So if you build a rad box, make sure you are careful about the distance from fan -> rad.
Optimal shroud depth is not a fixed number. It may depend on push/pull, fan size, fan speed, etc.
In my radiator mount (http://www.silentpcreview.com/forums/viewtopic.php?t=54589
), I used 55 mm distance, which seems to work fine (no, I didn't experiment with many distances) with slow RPM Noctuas pulling the air. Slow RPM and pulling rather than pushing is anyway good idea when you need silence. Extra heat radiation surface seemed also good idea for reduced airflow resistance.
My radiator doesn't keep any resonation noise. Possibly because it is hanging by 5 mm thick elastic cords with the fans.