Auto Air Conditioning & Heater Repair: expedition air conditioner, thunk, air conditioner

Question
When the a/c is turned on it initially blows cold air, then a "thunk" is heard and then starts blowing hot air. The sound is like an internal door closing.  Could it be the "dreaded door lock"?  If so, what is involved in the repair?
Thanks

Answer
Hello Paul, Sounds like the heater blend door is opening when a.c is on.Play with the cold/hot blend selector and see what happens.You may have a vaccum or electric acutator door control problem. These are dealer part  available only. Write back and let me know your results.