Grouop loop isolator - how it works

[amateur_25]

I believe I have the dreaded group loop issue.  My monitor's audio input is connected to my pc, there's this humming sound I can hear when there's no audio. 
It's not coming from my monitor because it's not there when I connect to my phone which is battery powered of course.

 I built up a simple circuit with an cheapo audio isolation transformer  1K:20K.
Yes it's not the ideal transformer but it was all I could get my hands on. My local rip off merchant maplin isn't the finest collection of components
Sadly I only had one 56K modem I could salvage the transformer from.

My question is what has the transformer done to my humming noise?  Am guessing it's 20K impedance that has affected the loudness of
the audio? Ah well you can't win. If I bought a ground loop isolator  I'd probably have the same issue unless it has a proper transformer.