Yesterday on the lake about halway through the day I suddenly developed bad electrical interference on my Huminbird bow unit attached to the trolling motor. I swapped head units but the interference remained each time the trolling motor was activated. It shows up as a blacked out portion of the water column from about 10 feet down to the bottom of the screen and returns to normal after the trolling motor is stopped. Since I haven't changed anything on my rig and it was working fine earlier in the day, any ideas where to start looking for causes or solutions?

Humminbird 727 with newer Motorguide 70lb 24v TM