pjazzman,
Welcome to the Side Imaging Forums!
Is there a second sonar unit running at the same time?
Does this happen when the boat is drifting and all motors and electrical devices are off?
Are you running only the HDSi transducer or are you running a two transducer configuration?
If two; which transducers and how are they connected?
Do you know someone else with a Humminbird unit that would allow you to test your unit on their boat/transducer? This does not have to be another 997 unit as any will work to test the 2D sonar on your 997 unit, except for those models with “3D” in their name. Just plug up the power and transducer for the test.