I have two words for you, son: Ground loop.
It happens a lot when there are connections between two pieces of
equipment when the manufacturers of each one haven't taken care of
internal grounding properly. Internet terminal equipment has always been
a prime suspect.
See if the hum goes away when you disconnect the Ethernet cable. Then,
when you've isolated the source of the problem, tear your hair out
trying to find a solution.
--
For a good time, call
http://mikeriversaudio.wordpress.com