I feel like it is probably related to one of the flaps in the AC box. It sounds like a flap motor keeps trying to close something. It seems to happen once ever 5-10 minutes and lasts a few seconds. Any of you guys experience this? I have a 91 zx

 

The closest thing to that I've experienced is my window auto-open/close button sticking. Once the window was at its extent, it would make this click sound every few seconds, took me a while to figure out where it was actually coming from. Do you have a full-roll-down function on your driver window?

 

The windows work perfectly. I've been down under dash and I can tell its coming from under there. I just cant seem to have my arm under there when it randomly clicks to find exactly where its coming from. It also only happens when the AC is on.

 

I believe its one of my Air Door Motors.