When I faced this problem, I wasn't able to find many good suggestions on Google, so I wanted to share my solution. It is unique in that it is an enclosure but it maintains sufficient ventilation to keep the equipment cool while preventing a significant amount of the noise from escaping. I primarily use it for air assist on my laser cutter, but also for pneumatic nailers in my shop.
http://www.artifacturestudios.com/blog/archives/985
Michael