Blend door must be stuck open. Loud whistling noise at 60mph and higher coming through dash. Very loud. 2004 GMC envoy

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

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If the bolts are from the blend door assembly, the blend door actuator could easily dislodge and jam. You will need to get in behind the dash and inspect the door actuator function. The actuator is new and shouldn't have failed already, so you might be able to salvage it and regain proper operation.

That is a electrically controlled actuator for the rear HVAC - heating ventillation an air conditioning system . Mode door or temp. door actuator . They are plastic an the gears wear an strip .
Loud clicking sound from rear of 2003 Trailblazer EXT
2004 Chevy Suburban Rear Air Temperature Actuator Blend Door HVAC You would have to do the same thing to replace yours .

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

I have to replace the actuators on Tblazers and Evoys all the time, usually the fresh air or blend door actuators but sometimes it ends up being the mode actuator which on some models is a little harder. Find which button on the control panel effects the clicking,usually made easier after the noise has stopped.Find which control button starts it back clicking

You should have replaced the blend door as well, they have an issue with warping and the noise is the door being broken free by the actuator, which will shortly fail working this hard to move the warped door. Have you ever heard of a kit to do this without removing the dash called "Heater Treater", google it.

It is likely that one or more of the blend doors or their actuators are stuck. Before any repair action is done, try allowing the HVAC system to recalibrate itself. To do this, turn all HVAC controls and switches to their OFF or CCW postions, then disconnect the battery for about 10 minutes. Reconnect the battery, but do not touch any of the HVAC controls or switches for about 2 -3 minutes. If this does not cure the problem, the HVAC system will need to be diagnosed. Access to the blend doors and actuators may require some disassembly of the center dash area.

The noise is coming from the rear HVAC unit, one of the actuators for the blend door is trying to move but is unable. Either the blend door is stuck or the actuator is bad.

Blend Door Actuator.Should have 2.One will controll the air across the heater core,and the other will adjust air flow direction.