My cat keeps coughing like he has a hairball...?

My cat's about four years old. In the last week or so he will periodically start coughing like he has a hairball but will not cough anything up. He's done this on at least four occasions. The coughing lasts for a minute or so and then he's fine. He's eating okay and is acting normal, but I'm still worried about him. I can't change his diet or anything because he's already on special food to prevent urinary tract blockages. Any ideas or insights?