I've done all this and I'm still getting noise when I run the motor:
Common ground from starting battery to negative post on 24v battery
Lowrance is wired directly to starting battery.
Clamp on ferrite chokes on all wires.
Transducer mounted on transom, with wire running down driver side, and tm power wires are running down passenger side.
2 things I haven't tried: using a standalone battery for the depth finder, and disconnecting the common ground and on board charger.
Any suggestions?