From the conditions you described, it's normal.

 

Thanks Solman- 9 years of ownership & never had that happen. It was an eye opener at the time..

 

I have been in other non GM vehicles with the AC on, and it never smells upon start up. Every GM vehicle I have owned or ive been in, always smells musky once turned on.
I even turn off the AC but let the fan keep running for about 5 or 6 minutes before I reach my destination before turning everything off. Cabin filter is new, drain hose not clogged, but it always smells.

bit off topic, sorry lol just venting

 

Maybe you should try the aerosol product made for the job.

 

Donbrew, yes I have tried something like that. It was a can that you press and it just keeps spraying until the can finishes. You have to turn on the recirc and close all doors/windows and leave it while the can finishes dumping its mist.
It lasted a bit but the smell came back after a few weeks. Maybe i need to find something more heavy duty

 

This one you spray foam onto the evap core, either through the center vent or through the cabin filter hole. It kills mold in the drain pan and on the core.