Welcome to the Humminbird Side Imaging Forums Spidey.
You can start with trying to give us a little more information here:
Is this the only sonar unit on the boat?
If not, does this happen with only your 898 unit on and nothing else on the boat running (including motors, aerators, etc…)?
Does this happen at all water depths?
Are you only using the stock XHS-9-HDSi-180-T transducer that came with your 898 unit?
Have you tried adjusting the Surface Clutter menu (both to its minimum and maximum settings)?
Do you know of anyone else with a Humminbird unit or do you have a second Humminbird unit, such that you could connect the transducer and power to you 898 unit and see if the same thing happens on the other transducer or not? This would not have to be another Side Imaging model unit as most will work for this depth reading test – just make sure it is not a DI model unit’s transducer. Test their unit on your 898’s power and transducer connections too.