Audio gound isolation should be enough, regarding noise induction from loops. Radio Scrap has isolation transformers on the peg if you feel the E/C one has bit it, these things are sturdy and I doubt it has failed without something visible damaging them.
You probably have ignition/alternator/electric motor noise on the power signals requiring filtration? Start with a simple toroid to see if it mitigates it somewhat, go to something serious like the CB filters repinned in the Yamaha connectors after that.
Thanks for the input.
There is something in the wires of the ground loop.
Too much time in a hot area and crammed in too tight.
Wiggle the plug or pull it out a bit and it is OK.
Plug in directly and all is OK (except for the noise.)
No fault to EC. It has been 5 years in a hot and unfriendly space.
I probably could fix it by putting on a new plug but think I'll order new.
Will order both from EC soon.