How to cheaply measure AC power noise

[Cbdb2]

A bad solder joint or dirty conector in a cable might cause that but if would only be one channel unless your using a stereo plug with a bad ground. We need more Info.

 

[Speedskater]

An Overview of Audio System Grounding & Interfacing
by Bill Whitlock, Jensen Transformers (retired)

https://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

 

[Audiofire]

Definitely good for permanent installation