What Causes Ringing In The Ear?

My two year old says there's a bug in his ear. The doctor says his ears are fine. He puts his finger in his ear And starts saying stop repeatedly. He does this on and off throughout the day for the past 4 days. When I ask him if it hurts he says no he said he hears music. I don't think he hears music but that he hears something. He fell out the car and hit the back of his head d so maybe that's causing him to hear a ring in his ear