Recently I decided that I don't like having a computer in my room that sounds like a jet turbine, so I've been on a quest to quiet this thing. First, I took a mesh grill off of the side fan on my case and that helped a bit. After that, I replaced the side and exhaust fans with a couple of Noctua 120mm 800 rpm fans and that quieted things substantially. Unfortunately, I still get a whooshing noise out of the exhaust. Is there any way to get rid of this without resorting to reducing the rpm of the exhaust or cutting out the metal of the back of the case that protects the fan?

Cut away! They do sell wire grills to protect your fingers if they get to close. http://www.petrastechshop.com/fangrilles.html

First thing I do to a case is get rid of the mesh behind the fans, even if the noise doesn't drop the CFM goes up so you can afford to reduce the speed a little. If its the honeycomb mesh then a sharp pair of cutters can be enough followed by a few mins with a file. Even if you don't get the perfect finish around the edges the grills hide it quite nicely

Cutting those mesh grilles out will get rid of "whoosh" sound, caused by turbulence. I remember doing this a couple of years ago on the TT Tsunami, it made a world of difference, the case was so much quieter afterwards.

Team that up with some silencing pads, you'll eliminate most of the noise caused by fans. Petra's has the pads, as well. http://www.petrastechshop.com/sovida.html

I have found that exhaust fans don't make anywhere near as much as intake fans do (don't know why..) My side fan on my case made this horrible sound, similar to a cessna engine, but quieter thankfully. Cut the grill out, took it to work and trimmed it up with a few tools, and lined it with some c-strip trim and a standard grill.. sexy.