Needs to be a pretty "soft" wall or it should simply reflect most of the noise? MikeC: Why do you seem to consider mufflers to be inelegant? (Or did I misunderstand you?) Can you elaborate your stance on mufflers?
Simple to put a piece of foam on that wall, especially when hidden from view under a desk.
Muffler -- when you get the intrinsic PC noise down really low, every bit of cfm is an issue, as is every bit of hot air you might trap in a muffler. If you can get the basic noise low enough, muffling doesn't really lower the noise, but it does tend to increase temperature, because chances are, your airflow velocity is so low. At higher airflow levels, I find that a muffler tends to give you more of a whoooosh "through a tunnel" kind of sound, compared to the more open shhhhhhhhh type of sound without the muffler. To my ears, the muffled sound is somehow more annoying than the unmuffled open sound. Maybe because the unmuffled sound is closer to white noise, which is more "natural" and familiar than the muffled sound. This is even with the muttled noise being somewhat quieter.
About a year ago, before I learned all the tweaks to really silence a PC, I built a massive sound containment box. It was built of 1/2"-1" thick medite board on a frame of 1" steel tubing. Some of the walls were "laminated" sandwiches, with damping material between 2 sheets of 1/2" medite
The entire interior walls were lined with soft foam. The main chamber held the PC. One muffler "tunnel" below the main chamber handled air inflow, drawing air from the bottom rear; another muffler "tunnel" above the main chamber handled air exhaust, again directing it to the rear, but the top this time. The cross-section of these tunnels was at least 1.5-2 times that of a 90mm fan.
It worked to help quiet the PC, which was already pretty quiet. I was hoping to get silence. That I did not get. Even with the absolute minimum airflow, you could here the turbulence of the exhaust air going through the muffler. I could get the noise down, but only by almost filling it with long hair lamb's wool (left over from my transmission line speaker cabinet building days). The problem was, the cabinet would slowly become hotter & hotter as the exhaust tunnel filled with hot air, then simply retained the additional heat coming into the muffler.
I finally got one of my other computers, a less powerful P3 (not a T-bird), essentially inaudible from more than 2 feet away. This was actually quieter than the massively enclosed T-bird system. At that point, I abadoned the box. It now sits lonely in my garage workshop. Maybe I should try and build a subwoofer around it so it's not completely wasted...