I have to say this is pretty cool, and sounds loud (and cheap to make too). One thing I might do differently is just “rough sand” the bottle, then spray paint it a nice color.
http://makezine.com/projects/bike-tail-pipe/?utm_source=feedburner&utm_medium=%24%7Bfeed%7D&utm_campaign=Feed%3A+%24%7Bmakezineonline%7D+%28%24%7BMAKE%7D%29&utm_content=%24%7BGoogleReader%7D