One of the best web sites for quiet computing is www.silentpcreview.com .
Here is a link to their recommended fan list:http://www.silentpcreview.com/article63-page1.html
Yes, you can easily swap out the stock fan for any one you want.
But, most of the noise from fans comes because they are operating at a higher rpm than you need.
And, yes, noctua makes some of the best. They also come with low and ultra low speed adapters.
If you undervolt the fan, or use something like a cheap zalman fanmate controller, it will be quieter at the price of airflow cooling.