Couple of questions.
1: Is this a bow mount fish finder?
2: Are you pulling power from the same battery that is being used by your trolling motor?
3: Do you have a transducer built in to your trolling motor or one that is mounted to the trolling motor?

Reason I ask is that your interference might not be what you think it is. It could be RF interference (radio Interference). You get this when you have electronics operating on similar radio frequencies in a confined area. I've even seen it from boat to boat. What you see on the screen might not be from the transducer when you turn the TM on, but rather RF interference because you turned the TM on.

I also ask the questions because I'm sure there is going to be a lot more people with way more knowledge and experience in this matter chime in.