Someone else searching for El Dorado. I've never managed to find a quiet air pump, although the reciprocating ones will always buzz while the screw type are generally quieter. The problem there is the screw type are generally much larger and used for fish house applications rather than just a small goldfish tank.
You could try a Tetratec APS pump as they're not too bad really. I have one in my front room for the tropical tank, and it's quieter than many I've tried. Having said that, I can't hear it over the background noise of the skimmers & circulating pumps on the marine tanks really.