The clicking in the furnace is the piezo spark generator that lights the furnace. The click in the AC is the compressor relay. There is a control board that activates the AC compressor. The control board contains a timer circuit that prevents the compressor from starting for a period of time since it last shut off. This allows the head ( pressure ) to equalize so as to not lock up the motor on restart. If the Thermostat calls for cooling ( relay click) and the timer has not timed out it the relay will click back out. This could happen several times if the AC compressor tries to start immediately after shutting off.
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