OK, I called HB support and they sent a ferrite ring to wrap the transducer cable around. I grounded the battery terminals together, and no improvement on the interference. Since it was much easier to wrap the ethernet cable (where I suspected the interference issue to be coming from), that's what I did. That did the trick, and I have good quality SI while using the TM about 98% noise free. I would think an additional ferrite ring would finish it off, as the ethernet cable is thicker, and I could only get 3 wraps on it, vs 6 on a td cable.