There is material to sound-proof your case, but I'm not mistaken, it doesn't work all that great. One thing you can try that's pretty in-expensive is to put some small rubber washers between the case and the fans. This will help stop the vibrations the fans give off, which end up vibrating the case.
If you do decide to get some lower RPM fans, take a look at the Panaflo series, which have great CFM for their low dBA ratings. Also, the Sunon low RPM/Ultra Quiet series are great fans as well. Just be sure to get ball bearing/dual ball bearing fans versus the cheaper sleeve bearing fans.