Coughing

I want to say maybe a couple of weeks after birth (he's 7 weeks now) my baby has been coughing. At first it was a single cough every few days or so, now it's a few coughs, sometimes a couple of times a day every. It doesn't sound like whooping. I'll be calling the doctor in the morning as he just did it again. Any idea what this could be? My husband just HAS to sleep with a fan on every night. Could this be a cause? Should a humidifier help?