Stridor is caused by

Stridor is noisy respiration produced by turbulent airflow through the narrowed air passages. It may be heard during inspiration, expiration or both. Inspiratory stridor is often produced in obstructive lesions of supraglottis or pharynx, e.g. laryngomalacia or Retropharyngeal abscess. Expiratory stridor is produced in lesions of the thoracic trachea, primary and secondary bronchi, e.g. bronchial foreign body and tracheal stenosis. Biphasic stridor is seen in lesions of the glottis, subglottis and cervical trachea, e.g. laryngeal papillomas, vocal cord paralysis and subglottic stenosis. Ref: PL Dhingra 7th edition of Ear, Nose and Throat; Pg no 333