Ac blows at feet and de frost vent not out front vents 96 corvette

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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It's the vent actuator, it moves the baffle that changes the direction of the air flow.
Here is a video of a broken vacuum hose that creates the same problem.

It could also be a failed actuator.

You most likely have a vacuum leak not allowing the vacuum operated vent/duct inside your instrument panel to operate. Check all of your rubber vacuum hoses in the engine compartment for leaks, cracks or breaks .

These vehicles came with 2 types of ac controls.
The vacuum control system uses vacuum to actuate the vents and ducting from hot to cold. A lot of times the HVAC vacuum line in the engine bay (black hard plastic) is either broken or disconnected from its vacuum source.
The electronic vent control: does everything at the control knob via vent actuators and the control panel itself. With this one it could be any of the components. You would have to locate them and verify operation by moving temp/vent selector.

the a/c control is electric for temp control and vacuum for mode control when a leak occurs it defaults to defrost floor .look for a leak or broken vacuum line first then control panel then mode actuator.

PROBLEM COULD BE BLEND DOOR ACTUATOR OR FAULTY AC CONTROL MODULE.

you may have abad slector door acuator.which is alittle motor bolted on the heater box.It move a door in box to direct air flow to right location.hope this helps.

Your last guess is probably the right one. The deflectors/directors are vacuum controlled and you may have vacuum line either broken or disconnected; these aren't fun to find. I would start under the hood and make sure the vacuum reservoir is intact (have seen them cracked) and then start tracing hoses from there.

you have a vacuum leak. also check your vacuum canister it looks like a round black ball some time is under the fender.

there are vacumn motors that operate these doors, control by module in dash, motor could have bad diaphram, correctly or module is not directing vacum properly