For info if you make up your own audio cables. Troublesome noise with instrument cables terminated in 6mm (mono) jacks, which I had attributed to the jack socket. The problem turned out to be intermittent connection between the centre pin solder tab on the jack plug and the centre assembly itself, despite the fact that the tab appeared to be solidly attached. I ended up putting a solder blob on the centre part and soldering the inner core directly there, that's solved it.
I've encountered a few instances of this in maybe 20 jacks purchased ftrom a hobby electronics outlet in Perth. No need to mention who, they'll all source their stuff from an OEM in China or somewhere similar.