Our heat/AC unit has been working fine since we moved in, over a year and a half ago. Today while I was doing dishes it started making this loud clicking sound, like a ticking clock sort of. At the time it started making the noise, neither heat nor AC was on, and the last time it had been used was almost 24 hours before for the AC. It seems to be coming from only one vent, and starting up the AC doesn't have any effect on it. Does anyone know what is causing this and how to stop it? Thanks for any tips 