hearing things
Having auditory hallucinations. Bob: "Listen, I'm sure that someone is right behind us!" Jack: "Bob, you're hearing things. There's nothing out here but us and the wind."
hear (or see) things
imagine that you can hear (or see) something that is not in fact there.ˈsee/ˈhear things
(informal) see/hear things that are not really there: So it was you that was playing the piano! I thought I was hearing things.(humorous) Tom’s washing the dishes — I must be seeing things!