Well, there are several things that you could do.
1. Buy quieter fans and install them instead. This could get expensive (or not) depending on the costs of the fans.
2. Buy a Rheostat Baybus and adjust your voltages as necessary. This gives you a range of voltages, instead of either 7V or 12V. A lower voltage means that your fans run slower and quieter, at the cost of CFM. This costs $35 at CaseETC. Link to follow...
Rheostat Baybus kit
3. Go to RadioShack and buy yourself a few rheostats, etc. and build the thing yourself. This costs less than $35. Heck, all I did was buy two rheostats. Forget the soldering, I just wrapped the wiring around the terminals. Forget the heatshrink tubing, electrical tape works fine. This costs about $10.
4. Do the 7V trick.