Open your hood, grab the plastic cowel in front of your windshield and lift it up and down to check for movement. This may not be the problem everyone is experiencing that has a rattle coming from the lower windshield. It was the culprit in mine however and this fix cured the annoying sound. It turns out the plastic cowel had some movement between it and the metal brace underneath it. It sounded like something was rattling or even knocking on my windshield when I hit cracks or small bumps in the road.
The fix: Simply lift up the cowel, carefully unsnap the five clips, and remove the two Philips head clips. Lift it just enough that you can get your fingers between the plastic cowel and metal brace. I then took some 1/8” thick foam cut into small rectangles with a slit in the center and placed them over each clip so it provided a soft barrier between the cowel and brace. By placing the foam over the clips and THEN snapping the cowel back in place, you place the foam between the two pieces and ensure the pieces will not fall out. Snap everything back into place including the two Philips head clips. This really tightened mine up and eliminated the rattle.