So, I've been researching methods of quieting air compressors and came across something a little different. There's a line of super-quiet air compressors carrying the GMC Syclone name, and a line of generators carrying the Sierra and Yukon name. This is not some rip-off, seemingly. It looks as though the names are used with GM's permission, and the equipment is made by a company called "California Air Tools."
http://www.gmcgenerators.com/
http://www.youtube.com/watch?v=JRzebltbLow
It appears these compressors are ridiculously quiet in comparison to all other direct-drive compressors on the market. At about $250 off Amazon it seems like a pretty good buy. I would buy one if I hadn't just picked up a new one myself.