Well, there is no way for anyone to know where a noise is without being able to see and listen to the car in person. If the noise is definitely in the engine compartment and only when the A/C compressor is on, I would be looking at the A/C belt, idler pulley and compressor and try see if I can confirm and isolate the noise to a particular part. It could be as simple as a loose belt.
I've reached out to more than one forum on this & you are the only one to reply. THANK YOU! I can better describe this sound by what one hears when you blow up a balloon, remove the opening from your mouth and hold it only tight enough to slowly let air release. I think it is the diaphragm in the "fast idle actuator". I would like to start by replacing it but so far have been unable to source it. Any additional thoughts? Thx.
Good luck finding that! Try the Z-car forums, I guess. Salvage yards, but I wouldn't hold my breath! Most of those old 280ZX's that were brought to salvage yards have either been crushed or rusted into dust by now! But, you never know if one may be sitting in some dry, desert junk yard in the southwest... You might try Car-part.com to locate one. Finding some parts on older Nissans and Datsuns can be like finding a needle in a hay stack!
Thx for getting me to think salvage yards. Would you believe that I found a rotting ZX in a yard here in South Jersey. After fighting to get the hood up, there it was - a battered FICD Actuator. It cost me eight bucks and works like a charm. I was pressed for time but will return to pull whatever else I might need. Thx again.