Rfi is a common issue.
Are you using the correct trolling motor transducer.
Make sure all graph cables do not come in contact with trolling motor power cables
Disconnect onboard charger from cranking battery
Do not connect graph to trolling motor batteries.
Check with Humminbird and see if they will send some ferrite rings.
May also have to ground the trolling motor.
These are just some of the common issies. There is no one fix. It may be a series of solutions that will solve it. Bit make sure you test on the water. Always test in the water. Testing on the trailer will not give you the water grounding element and can give you different results.
This link will show ya how to ground trolling motor.https://www.humminbird.com/faqDetail.aspx?id=3660