I looked into sound proofing once, it turned out it was actually cheaper to buy a proper compressor than sound proof my workshop ( the one I had at the time was about 30ft x 7 ft ).
These are probably a bit spendy for doing toy cars but they are super quiet, like a fridge, literally, you can sit next to it on the phone to a custom and they'll hear everything you say.
http://www.everythingairbrush.com/ac...mpressors.html