I would also suspect the motor mount ...
You could further experiment with the transducer on a pole by moving the transducer aft to verify if the lines go away in the SI image ...
If the lines go away at just a short distance aft of the transom, you might could add a transducer mounting board ...
Another option (but more expensive) is buying a second transducer and using the Garmin 12 pin Y cable ...
Using a Garmin Transducer Y-Cable | Garmin Customer Support
This Setup allows left SI to come from a port mounted transducer - and right SI from a starboard mounted transducer ...
Rickie