The inferior Colliculi play a major role in hearing. Since the loud noise was what prompted the response it is safe to say the inferior Colliculi is responsible.
Neurons in the inferior Colliculi respond to cues such as intensity, the difference red in arrival of sound to both ears.
However, the head snaps up because the information from the inferior Colliculi is transmitted to the superior Colliculi which is involved with movement such as the head.
