James Perrett wrote:Your older PC probably has different internal grounding so it is a bit of a red herring to say that it worked before as that was a completely different situation. You are going to need to break that ground loop somehow and purchasing an isolation transformer is probably the best way to do it if you don't want to go making up odd cables.
I totally agree with this - sometimes you can have multiple ground loops and no digital noises in the background, and then change or add just one item of gear and have a hell of a time tracking down and removing all the ground loop offenders before you cure the problem.
You just have to be systematic and work through them one by one until you have a clean system. Here's a Q & A I wrote some time ago that describes the step-by-step process:
https://www.soundonsound.com/sound-advi ... -my-studio
Hope it helps!