Pull the bars and stem. Remove the handlebars. Clean the bar clamp area, ensuring that there are no burrs, scratches or dirt above nominal level (no bumps or protrusions of any kind). Clean out the inside of the steering tube, carefully and clean off the stem, carefully. Once again, look for and remove any high spots.

Now, assembly the whole works and use grease in the steering tube to stem contact area. Install the bars dry in the stem clamp. Ensure that everything is tight.

Not sure if that will do it, but it will be one way to eliminate some of the possible causes of the creaking.