2/13/16 The horn on the Plymouth Road Runner sounds (or sounded) like Warner Bros.’ Road Runner character.
It was an officially licensed sound, and you probably had to push the horn twice in quick succession to get the beep-beep.
There is dispute about whether Road Runner was saying beep-beep, meep-meep, or mheep-mheep so I looked until I found an official Warner Bros. listing:
Road Runner is a happy bird who can’t fly but travels desert highways like a bullet train. He’s innocent and carefree. With long legs, graceful tail feathers, and a cheery “Meep-meep!” he’s always way ahead of Wile E. Coyote. When you can run at the speed of sound and make an instant U-turn, no one can catch you. That’s genius.
If you have no idea who the Road Runner is:
If you’ve ever wondered whether it is Warner Brothers or Warner Bros. (as I did when I had to decide between Warner Brothers’ or Warner Bros.’):
Warner Brothers, in full (1923–69) Warner Brothers Pictures, Inc., or (from 1969) Warner Bros. Inc., American motion-picture studio that introduced the first genuine talking picture (1927).