You are getting RFI from the trolling motor. Nothing unusual about that.
Contact Humminbird Customer Service for help with that issue.
Give them more information than you have done here. What motor and how is the sonar unit powered.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, SOLIX 12 SI, 360, RC-1