Get one of these. I have a 49CFM rated one right in the front of my case and I can't even hear it. It's quite powerful too. And there is also a 63CFM rated one. Just came in, the new 75CFM rated one too.
There great fans and for the price, you don't have to worry about replacing them, unlike Sleeve bearing fans where they have a shorter lifespan and are rather loud.
One day I even checked if the fan was actually spinning it was so quite. I took the front cover of my case off and saw it right there, spinning pleasingly fast. Took a piece of paper and it stuck right too it.
Edit: Forgot the link: