I wouldn't change any thing.Hook your finder up to one battery(make sure the wires are not next to your motor or boat wires keep them as far away as you can)then switch to the other battery to start the motor and see if the noise goes away.If not you have to try to find out where the noise is coming from,so hook up to the new battery see if that works if it doesn't double check to see if your ducer is mounted right.It should be away from the big motor,it should be away from as many rivets as you can if the boat is an aluminum(just make sure it not right behind a skate on the bottom of the boat)and it should be 1/8 inch below the bottom of the boat.That should be it if not there might be other issue.But do it by process of alimentation.Between every one on this site we should get you up and running.Just my 2 cents.Somebody might be able to fix your problem first time but This is the best I can give you with out seeing the boat.Hope this helps.Good luck.
If you determine that the transducer is not the cause of your interference, you can try placing a ferrite choke core on the power lines, positive and negative leads, before you rewire. Radio Shack also has a closed ferrite that you can wind the power leads on. I have used these before and was effective, but every case is different. Ferrite Core
If you get to the point of rewiring,try to wire the Sonar positive to the Battery Switch terminal that the Battery Switch feeds the console and wire the negative to the battery, this way you still have the function of the Battery Switch. If that does not work than wire it directly to the spare battery.
