I have a 2007 Chevy Impala. When the car is turned on and the AC/Heater is first engaged, there is a terrible smell that emanates from the vents. It smells like a musty port-o-potty (I may exaggerate) and promptly fades after a few seconds. Please let me know what this could be and how to fix the problem.
Apart from a dead rodent in the ductwork you may have semi plugged fresh air vents.
Remove the plastic vent screen and clean out any debris that has accumulated there. Old leaves, dirt, etc)
Make sure the drain hose is clear and also replace the cabin filters (if equipped).
This may help and may not depending what is causing the stink.
Let us know how you make out anyway.
In addition to what Roadrunner suggested, do what your owner manual probably suggests and leave the HVAC system in any setting but OFF or MAX/recirc when you park it.
Post back if the problem continues