that is the AC, I have noticed the same in my car when I run the defrost. My advice is to use the defrost every day when you drive the truck on high heat /low fan, and that will help get that mildewy smell away. Because technically the AC mixes with the heat when you use defrost that smells tends to produce some moisture in the ducting.
I note this more in my car as I park it for the winter, so I just run it for about 20 mins with the defrost on full hilt during the winter every month or so. Pretty much takes care of h mildew smell.