Hey Marshall Forum!

About a year ago I bought my first ever Marshall amplifier; the Silver Jubilee 100W head (with 4x12 cab).
Since I use it mostly at home, I haven't cranked the volume all that much. But when I do, I notice a loud humming noise.

Today I turned the amp on, without having the guitar cable attached to it, cranked up the Master volume (with 0 gain!) and it hummed like crazy. 
What could be the source of the humming?? And more importantly, how can I get rid of this awful humming? 

The only cables that were attached were the footswitch, FX in/out and the power cable.