travish10,
You need to call the Humminbird Customer Resource folks about this. 1-800-633-1468
They will have some suggested fixes for you to try. Be forewarned though: what works on one boat does not work on all boats.

What you are seeing is RF (Radio Frequency) interference from the Pulse Width Modulation (PWM) circuitry in the trolling motor. This circuitry is used for all but full speed so that is why you don’t see it than.