This is definitely interesting. This definitely sounds like trolling motor RFI, but it could be coming from the Pulse Modulation that Minn Kota uses for speed control. If the RFI goes away when the trolling motor is at 100%, then it is probably the Pulse Modulation that is causing the RFI and not the motor itself.
Please try the 100% speed test to see if the RFI goes away and let us know.