Grounding the unit to the trolling motor also makes all the difference sometimes.
To do this, run a wire down the shaft of your trolling motor just as you would the transducer wire. Then connect the wire to the trolling motor skeg as best you can. Sometimes there is a bolt there; if not, drill a small hole through the skeg and use a small bolt and nut and a wire connecter there. Make sure the wire connector or the wire itself is making contact to the metal of the skeg. You may have to scratch off a little paint. Use a continuity tester to make sure that you are grounded. Now make sure you have leads directly from the non-trolling motor batter all the way to the front of the boat. Don't tie into the dash, the front panel or any other unit. Two wires from the battery directly to the front graph. Be sure to insert a fuse near the battery. Wire the hot wire from the battery directly to the positive on the unit. Then tie all the grounds together--the ground from the battery, the ground from the battery and the new ground from the skeg.
This will eliminate interference always*.
*Disclaimer--this works 90% of the time always.
That's one of the solutions I tried with mine and it actually made it better, but still had some interference.
I would think its possibly associated with the transducer wire and the power both being in close peroximity inside the shaft