I have a 1997 Eldorado, and this noise comes on only when the a/c is on. It also happens when the car is on, or off. It sounds like a pissed off cat or a low growling noise. Any ideas of what this could be?
You say it does make it when the car is off? Is it for a few seconds after you shut off the car when the a/c was running?
The only thing I can think of that would make a noise with the engine off is refrigerant flowing through the orifice. This is a normal condition with any CCOT system. It is possible, though, that you are slightly low on refrigerant. I would have someone check pressures and evaluate the performance.
But that noise I don't think would be described as a "growl" more like a "hiss." See if it's your blower motor--does it do it with the "ac off" mode selected?