Check it at another location. It could be that your power is not good. 50Hz hums are often caused by bad earthing. But as there are so many things that can cause it, a good diagnostic run down may help isolate the cause.
ie. Does it hum:
with or without the instrument connected;
when the volume on the instrument is turned all the way down;
at another location;
when flourescent lights are turned off;
when the volume on the device is all the way down (etc.)