2002 Mountaineer heater noise

I have a 2002 AWD V8 Mountaineer with manual heater / AC controls. When I change the temperature control it starts making a popping noise like what ever is trying to move inside the unit is stuck.
Any ideas what it would be and how difficult to fix?

Could be a bad actuator motor on the a/c-heater housing blend door. The shafts have been known to crack and then it cant find a park position so it keeps rotating and hunting. It's a bear to access on the a/c heater core housing behind the dash panel.