http://www.parkswhistles.com
These whistles can be very, very quiet, or adjusted breathy to full on. They are great for private practice or for small to medium sessions. You can easily carry one in your pocket or purse. They don't weigh much at all either. Check it out.
Regards,
Carey
PS: A musician in Holland posted this information on his web site where he describes the instruments he uses. Nice photography. http://www.rycooder.nl/pages/guitar_parks.htm