Chasing the source of interference like that can be very frustrating and there is not one common solution that fits everybody. It took me a couple of months to resolve it the first time I ran into the problem. Although I am familiar with HB, I imagine the Lowrance protocol should be similar. As mentioned above, there are many suggestions available, but you can start with simple things first, like software updating, direct wiring, isolating the cables, noise filter adjusting, checking the transducer cable for nicks, checking the fuse in the head (if newer Fortrex), and unhooking the battery charger before fishing. If none of these are successful, later on chokes, ferrite rings, grounds, etc. can be tried. Sometimes it will be a combination of some of these. I agree that it would be a good idea to put a post over on the Lowrance forum on BBC to get some direction.