For instance this sleep sheep, produces the sound of ocean surf, whale song, mother's heart beat and spring flowers, with adjustable volumes. (newbornbabyzone.com)
By pumping background data through a computer, they can produce sound waves. (newsweek.com)
Just a thought, have an assistant rocking the car from the side may very well produce the sound when the vehicle is stationary. (mechanics.stackexchange.com)