How can I reduce whistling due to wind across a gas fireplace vent?
The first thing I would check, is to make sure there are no blockages in the vent. If there is debris restricting the flow of air, it could produce noise.

Another cause of this could be that the wind is blowing just rught across the top of the pipe causing it to vibrate, similar to blowing across the top of a bottle to produce sound.  To prevent this, you could try cutting small V shaped notches around the perimeter of the end of the pipe. 



Cutting the end of the pipe at a slight angle (2-5&deg; &lt; 90&deg;), may also prevent the wind from crossing the end of the pipe in a way that produces noise.