My favorite is a jade shepherds whistle handcarved by a Maori craftsman from New Zealand. They aren't cheap and they require some practice to get the sounds you want. I tend to hand it from the rear view mirror and play "whistle to the oldies" when my dog's not in the car. If I can follow along with Bohemian Rhapsody, I can make about any sound I want. You can also get them in metal or plastic for a whole lot less