My air conditioning unit is five years old and was professionally installed. My house is 45 years old. This cooling season, a new sound has developed in one room in the house. It occurs when the AC is running. The sound is a droning/humming sound coming from the vents. The closest sound to compare is semi tires on asphalt. I only hear the sound in this specific room. Don't know if this makes a difference but the room with the sound is the room furthest away from the outside AC unit.
Any ideas of what is causing this sound and what I can do to make it go away?