In my experience this problem is caused by a ground loop in the USB or Firewire connection
between the interface and computer. Sometimes it can be solved by running the interface
from a PSU rather than bus power, and with laptops if often goes away if you switch the
computer to battery rather than mains operation. Sadly, sometimes the only fix is to
change the interface, because although interface designers generally blame the internal
grounding inside the computer, some have managed to adapt their designs to avoid the
problem while others have not. As a rule this type of noise only affects the monitoring
and doesn't come out on the recording so if not too loud you may be able to live with it.