I have the exact same issue with a MacBook (2015). As soon as I plug in the audio jack, I will hear background noise, except if I touch the aluminium body. I so

I have the exact same issue with a MacBook (2015). As soon as I plug in the audio jack, I will hear background noise, except if I touch the aluminium body. I solved the problem by replacing the default short connector to the socket with a desktop-like long power cable, that unlike the short one has a ground connection.