Low Mains hum

Hi there  I have a problem with my A/D converter. I am running this through a DIGI 003 and there seems to be no problem with the system linking up they both talk to each other fine the signal goes through the system from all of the inputs. However the problem I seem to be having is there is a low hum which sounds like mains hum I have had a look inside and the unit seems to be earthed ok. The mains hum seems to dip a little when turning on the phantom power and sometimes just  dips on it's own. I thought it might be a dry break on the circuit board but I would rather leave it as a last resort to remove it as it is a lot of hassle. Would you have any idea as to what else it might be. As everything is working fine i'm totally stuck.