Why do I have a musty smell when cooling but not heating?

Question

I have central AC and furnace. They're both high efficiency units from Lennox. It's an upflow furnace. Lately when I turn on the AC I started noticing musty smell. But when I change to heat, it goes away and comes back once AC is on. Would cleaning the evaporator coil rid of the issue? Heat would also pass the coil, so I'm not sure why it's only smelly when AC is on and not when the heat is on. I wanted to ask here first since I haven't really figured out how to clean it yet. The outside of the coils look clean, but since it's upflow I would imagine it might be dirty inside.

Answer 1

Moisture.

When heating, everything is dry and there isn't much evaporating into the airflow. When cooling there's condensation, which dissolves gunk on the coil and allows it to evaporate into the air. It also grows more gunk.

A cleaning is probably in order, but there may be other issues resulting in excessive moisture buildup.