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.
Erin
