A sound added to the key up of a CB radio. It is also known as a "key up tone". This tone can be (and not restricted to) an ascending tone, a tri-tone or a sound clip. It's uses vary by the user. Some are used to glamorize one's radio while it could also be used to annoy other chicken banders. Douche whistles are most popular with truck drivers and Cb'ers in Mexico.
Did you hear the douche whistle on that Mexican's radio?
That guy is annoying the shit out of the truckers on 19 with that douche whistle.