I have been running my heat pump on my Trane A/C system. I t was a warmer (& more humid) day so I had to run the A/C on cool for awhile this afternoon. There is a smell coming out of the ducts that pervades the house. It isn't present when I run the heat. What is it? And what can I do? To stop it.

It is dirty sock syndrome. The interior A/C coil needs to be cleaned. Most of the time you have to take the unit apart to do it. Unless you are real handy with tools and know what you are doing I would leave it to a professional HVAC company. They can get chemicals that do a much better job than bleach or 409.