SO the LSS1 TD is connected to the unit that is good, right? And the one that is bad is sharing it?
So the noise must be coming in on either the ethernet cable, or coupling thru the power. What do you mean by "on the same set of wires"? You mean the power is daisy-chained with only one pair going to the battery? Just spit-balling here, no real Lowrance experience. $0.02. If possible, while up and running on the water, create the noise (TM on, etc), and while watching the noisy one, temporarily unplug the ethernet and see if it clears up. Obviously the shared sonar pic will go away, but we are interested in the noise. How is it foggy? You mean freckly kinda noise on all screens (views)?
"Alive without breath, as cold as death; never thirsty, ever drinking, all in mail never clinking."