And one more, the Panaflo M. Similar to the other 38mm, it tends to have a lower tone overall, but a more pronounced grit motor like noise as well when undervolting.

This gritting motor noise is why I generally like the 25mm fans a little better, but it is a trade off for a higher pitched noise.

Panaflo M = Panaflo FBA12G12M
http://www.youtube.com/watch?v=fqMWeTYLrD4