Liberty E-bike - This is a video of a Liberty 48 volt city electric bicycle. (LINK)
This bike appears to be using a "brushed" hub motor. And it appears that brushed motors tend to make a humming sound. Most "brushless" motors seems to be far quieter. (Also, see the Liberty Electric Mountain bicycle) [Cycle Santa Monica (blog)]
This is a thread on the Zappy list about the original manufacturer of the E-Max .. the chinese company that made the E-Max is in business and visible on the Web at http://www.topmotorx.com ... ZAP is planning to import and sell it as the Zapino.