Here is your problem. Your reference ground for the u2 is the TM ground. Your Sonar is using the starting ground. There exists a ground loop between the two batteries. This ground loop causes stray currents and this is amplified more with metal boats. Tie the start battery negative to the trolling motor battery most negative. This should improve things. If you lived close by, I would fix it for you.
Member BS Pro-Staff and Billbob Pro-Staff
Proud Member of Team Geezer... authorized by: billbob and "G"