Check your signal path to make sure you're not clipping (overdriving) the input of your amp or mixer. We like to start with the guitar's controls in the center detent position.

Try fresh batteries. The batteries will drain down to the point where they barely have enough voltage left to power the electronics. Just before they die completely, the guitar will still be trying to power up but will have a weak and/or distorted output.

The Expression System 1 uses magnetic Dynamic Top Sensors, and anything metal that falls inside the guitar can bounce around and end up stuck to a top sensor, which can create noise or distortion. We regularly see pieces of broken guitar strings with the ball ends stuck to sensors, as well as steel wool fragments and twist ties from new cables. Look inside the guitar with an inspection mirror, or gently feel the top sensors, to make sure nothing is stuck to them. If you find something, simply pick it off to remove it.