When I am running the A/C portion of my central HVAC, there is a slight musty odor coming from the vents. However, when I'm running the heater portion, there is no odor. The best I can tell is that the furnace and the evaporator are in line in the unit in the attic. This unit is only 4 years old and the evaporator coil are not accessible. Any ideas on what may be wrong or what I can do to eliminate the odor?