A baby who cannot relax can be helped to do so by a variety of constant rhythmical stimuli. it will work if the trouble is some kind of general and diffuse irritability or tenseness which is preventing a tired baby relaxing into sleep. The burring sound of a fan or heater works excellently. So does the sound of a car engine.

Your Baby and Child (ed. Knopf, 2013) - ISBN: 9780307594426