In most cases, a steady hum or buzz at the output is caused by grounding issues between the Apogee USB interface and the connected equipment. Because bus-powered USB interfaces are grounded through the USB and audio connections, and not through a 3-prong grounded AC connection, the situation may arise where the grounding configuration of the connected equipment must be modified. Check that the computer is grounded with a 3-prong AC connection.
Some experimentation may be required to find the grounding configuration that results in the lowest noise. It may be useful to disconnect all equipment except the Apogee interface, the Mac and a pair of headphones, and then connect additional equipment one piece at a time to determine the source of the grounding issue.