Is it possible to run an extension cord from another room (ideally on a different power circuit) and plug the two into different power supplies?
Another possibility is interference on the antenna feed, try having both units connected and unplug the antenna fly lead from the STB and/or wall. Still humming?
One other thing to try is to get hold of another STB from somewhere and see if also exibits the same symptoms.......
It seems that the answer is the interference on the antenna feed, with both of them plugged in the humming stops when I unlpug the antenna. How can I fix this mate ?