If your fan guards are the kind with tiny holes or even the kind with small slots, you can make the fans quieter by cutting them out and replacing them with the kind that look like barbecue grills.
http://www.svc.com/finguargril.html
Front fans don't need any guards.
This will do 2 things increase airflow, and slow the fan down 1 or 2 hundred RPMs.
I know this sounds counter intuitive, but although not true DC fans, the speed is still effected by load, the more airflow the more load. hence slower speed and quieter.
Edit: the little holes are a source of noise also