What is the strange gulping/swallowing noise my toddler keeps making?

Air swallowing: Toddlers do the oddest things... sometimes they do get into a habit of "air swallowing" and then belching. Often if they notice a reaction in your face to the habit, it will reinforce it and they will do this more often. Ignoring is the best. Sometimes kids will do this in response to heartburn or reflux symptoms, so it is a good idea to have an evaluation by your pediatrician to investigate.