Hey Marc,

Yes. Rock.

Mogami W2524 is very good cable, sheilded well. Dispite what you may read, makes a huge difference. Expensive to buy, not bad if you make your own (i do)

Yes, it can take time to track down what can be in several areas.

Do you have a chain in the loop? Try bypass that.

Also, a ground loop in the ac could be the cause. If you dont have a device with a ground lift, you can try a different outlet, or better yet, a differet path to breaker being sure to unplug other components from the loop to check... good luck!