Hi Stan,
Your whistle sounds a little "breathy" to me. According to a reference I have here, that could be caused by your windway not being focused enough on the edge at the top of the aperture. This book recommends the stream of air (or steam) be about as wide as the aperture, and probably less than a sixteenth inch thick. The 'thickness' of the stream should hit head-on at the upper edge of the aperture, with the edge cutting the stream roughly in half.
The shape of the aperture also has an effect, with short wide openings producing a clearer focused tone, and long, narrow openings producing a breathier tone and requiring more blowing pressure.
Reference: Musical Instrument Design by Bart Hopkin