Mine clicks during startup and when changing temperatures. It is the switchover valves for the Tempmatic system. I bought a new unit and it is on my "To Do" list. Hopefully it will solve my occassional hot air coming out of the side vents during AC usage.
Same thing on mine. It recently started making that noise, and usually when the thermostat is in the 70-75 range and the car switches between slightly warm to slightly cool air. When it's hot out and I have the A/C on and the thermostat is low, it won't make that noise. It's one of those things I have to get to, but I'm a pretty big dummy when it comes to climate control related items.
If the "clicking noise" is a constant chirp, then it is likely to be the aspirator motor/fan. As the motor ages, the bushing inside the 12v motor start to bind and emit a chirping noise, sort of a smaller version of the noise that older blower motors make.
Located behind the glove box, the small electric motor driven fan creates an air current via a tube that runs up the the nearby A-pillar that terminates near the climate control's temp sensor mounted up between the visors. The air current is used to get an accurate reading of the temp inside the cabin.
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