The mysterious knocking that wasn't a person

Three nights in a row, exact same time — 2:17am — loud knocking on our cabin wall. Not the door, the wall. By night three my husband was ready to fight someone. Maintenance came, checked everything, found nothing. Night four: silence. We never figured it out. Mentioned it to our steward on departure day and he laughed — apparently that section of Deck 10 on Harmony has a ventilation pipe that occasionally does exactly this when the ship's air pressure changes in certain weather conditions. 'The ship is just talking,' he said. The ship was being very rude.