If it appeared when moving cables it might be a loose or bad cable. Systematic removing connects until you isolate the problem is usually the best method. Or remove all of then and plug them in one at a time. Tedious, but sometimes it is the only way.
I had a similar problem once and it took a long time to find. Turned out to be RCA connectors where the back of the connector screwed on. It was loose and apparently broke the ground. When I tightened it up the ground loop went again. I replaced all the cables of that type and have not had any trouble since. I would suggest you check all your cables.
I had a similar problem once and it took a long time to find. Turned out to be RCA connectors where the back of the connector screwed on. It was loose and apparently broke the ground. When I tightened it up the ground loop went again. I replaced all the cables of that type and have not had any trouble since. I would suggest you check all your cables.