Before you get a new fan, why not try undervolting the one you have? It's a pretty simple mod if you just want to make one fan run quieter. It's easy to do, especially if you have a 3 to 4 pin adapter lying around. Just pop the pins out of the molex connectors (jeweler's screwdrivers work well for this), and switch them around so that you are delivering 5V or 7V across the fan. Refer to this article
here on SPCR for a little more detail.
If you don't have an adapter lying around, you could just use some insulated wire. But I'd recommend buying the adapter because it will be a much neater job. You can usually get them at places like CompUSA, or from local mom-and-pop vendors.