The reason for the humming noise is improper grounding. Audio signals are low voltage level AC signals (Over simplified). An audio signal could be for example  1V, 1 KHz signal. When their is improper grounding, low level noise signal example 10mV 50Hz can get coupled to the audio signal causing humming. (Values are made up)

I would suggest installing shielded cabling which will provide good grounding, that essentially is the passive circuit that you are seeking. 

An audio signal at specific frequency with improper grounding (with noise) would look something like the below signal. 

[![enter image description here][1]][1]

An audio signal at specific frequency with proper grounding (without noise) would look something like the below signal.
 
[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/2ZM72.png
  [2]: https://i.sstatic.net/v95KO.jpg