I had that issue years ago with my old MK Powerdrive 2 and a HB 575 DI unit... it took several ferrite cores to fix the problem. My HB 999SI came with a pre installed choke on the power cord. My Garmin has something similar.
Some good info on the subject here...
RF Interference in Trolling Motors - AAARGH! Sonar Noise! by Adam Broughton