Where is the mode door and the electric actuator motor located on a 2004 GMC ENVOY SUV

The knocking is probably the blend door actuators. They have been detached or become stuck. They operate the doors that direct the flow of air.

https://www.google.com/search?q=2004+gmc+envoy+blend+door+actuator+location

..

That's the mode door , not blend door . electric actuator motor is bad . replace it .

There isn't a heater control valve in the hoses, it controls the heat in the vehicle with a temp blend door which has an electric actuator.

There are no vacuum lines for the HVAC , electroniclly controlled .

Mode Actuator
The mode actuator is an electronic stepper motor with feedback potentiometers. The HVAC control module sends signals to the mode door actuator through the mode door control circuit. Zero volts drives the actuator in one direction while 5-volts moves the actuator in the opposite direction. When the actuator receives 2.5 volts, the actuator rotation stops. A 5-volt reference signal is sent out over the 5-volt reference circuit to the mode actuator. When you select a desired mode setting, logic determines the value of the mode actuator signals. The HVAC control module's software uses this reference voltage in order to determine the position of the mode actuator through the mode door position signal circuit. The motor moves the mode door to the desired position.

Defrost Actuator
The defrost actuator operates the same as the mode actuator. The HVAC control module sends signals to the mode door actuator through the defrost door control circuit. Zero volts drives the actuator in one direction while 5-volts moves the actuator in the opposite direction. When the actuator receives 2.5 volts, the actuator rotation stops. A 5-volt reference signal is sent out over the 5-volt reference circuit to the defrost actuator. When you select a defrost setting, logic determines the value of the defrost actuator signals. The HVAC control module's software uses this reference voltage in order to determine the position of the mode actuator through the defrost door position signal circuit. The motor moves the defrost door to the desired position.

Here's a video that may help. GMC Envoy HVAC actuator Replacement

Removal & Installation

Blazer, Jimmy, Xtreme & Bravada
The mode actuator is located on the top left side of the HVAC module.

1. Disconnect the negative battery cable.
2. Remove the knee bolster.
3. Remove the right side driver knee bolster bracket from the steering column bracket and the intermediate bracket.
4. Disconnect the vacuum connectors from the mode actuator.
5. Remove the actuator rod retaining clip.
6. Remove the actuator rod from the mode door lever.
7. Firmly pull on the mode actuator in order to remove the actuator from the HVAC module.
   
   Is this what you were looking for

More than likely stepper motor that operates door is defective. Try tapping on module that controls the door. Dash control may be defective you need a DVM to check electrical power at these outlets, do not use 12 volt test light.

DVM: Digital Volt Meter

If you recently had a weak battery or replaced a battery the HVAC unit will perform a recalibration of all doors. This causes the door motors to be moved full cycle and the mode door will stick some times. Usually this is the problem. The actuator motor will need to be removed and replaced...sometimes they can be recalibrated, but not often.
Good luck, i hope this helps.

door lock actuator solenoid ( inside of door) needs to be replaced

You will need to pull off the back panel to access the handle and rod. You either have a broken handle, or the rod that goes to the actuator has fallen off.