The whistle does not work, any ideas on a fix?

A vacuum leak would cause the check engine light to come on and throw a code. It is more likely mechanical. Ideas in the link.

https://www.google.com/search?q=2005+Chev+Avalanche+Z71+-+whistle

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A whistle noise is usually a vac leak.
The cap is where you fill the tank with coolant.

What type of power source is it hooked to? straight to car battery or some other power source? Sometimes cart alternator noise is picked up by radios. Try hooking the power source up differently

If it's a whistle, or chirp like sound, then find what is rubbing together to cause the noise. If it is a low pumping sound that seems to be coming from internal to the compressor, then that is the way they sound.

I might caution you, if the tech's that are putting R-12 in the system are doing so through clamp on type service ports, they are violating the EPA mandate for the type of equipment used to service A/C and refrigeration equipment. Besides the fact that it will continue to leak because the type of fittings used will not hold R-12.

you need to replace it with a new one it will be corroded. heat kettle then drive out pitch pipe with a screw driver from below, insert new one from above.

if it's not running bad, it may be a simple crack in a pcv line, or a loose hose(vaccum). should be simple to fix.

Have you tried descaling the machine?

You may be having problem with air scaping from the base of the hum. it will probably may have to put silicone around the base of the hum. to stop the air scaping.

Does the whistling show up no matter what you have selected for an input?
Disconnect all the inputs to the receiver and try listening again.
Let me know.
Thanks

There is a vacumn hose that has come loose somehow. Use a second person to isolate the whistle to lead you to where the hose is cracked, loose, or detached. Fix or replace and this should fix this whistling sound. Hope this helps.