I'm going to through another possibility out there. Mine was doing that last year when I bought it. The next time you notice it doing that, look at the small lights on the Heat/AC Controls. One or more of them is probably blinking in unison with the "clicking" If that is the case, then the Control unit is about to go bye bye. I hade mine replaced. After the clicking started, the Recirulate function stopped working, then the fan speed control went crazy(Couldn't turn it off), then I couldn't turn the AC Compressor off. This all happened gradually over about 2 weeks time. If it's out of warranty, you see them for fairly cheap on ebay. These units are well known for flaking out.