Is it whistleing the star bangled banner? hehe.
ohmygoodness >.<
there's a couple things you can do for a whistle. Think of whistling with your mouth/lips- if you stick your tongue in the way, or change the shape of your lips, the whistle will stop, but won't affect the airflow out of your mouth. Same thing goes for the airhole in your atty.
personally, I use SS mesh to stop my whistlers. Just take a little bit, roll it up, and put it in there. It only takes a little.
You can literally use anything, but it's a balance between stopping the whistle and not affecting airflow. be creative.