Causes of stridor 

foreign bodies

infections – croup, epiglottitis, tracheitis; retropharyngeal or other deep neck space infection

anaphylaxis / angioedema

laryngeal or tracheal trauma

vocal cord paralysis

lesions of the vocal cords – benign or malignant

In very young children:

laryngomalacia

subglottic stenosis

vascular rings

Be extremely cautious when diagnosing croup in children under 6 months or over 4 years, as it is extremely uncommon at these ages.