DIAGNOSIS: A consistent, quiet clicking is the internal relay switching. This is normal operation when it's calling for heat or cool. A loud, erratic, or continuous click when the system is *off* is not normal.
FIX: If the clicking is happening when the system should be idle, you likely have a wiring issue causing a short or the relay is stuck. First, remove the thermostat from the wall plate. If the clicking stops, the problem is in your HVAC wiring. If it continues, the thermostat itself is faulty.
IF THAT DIDN'T WORK: For wiring issues, inspect the wires at the wall plate and at the furnace control board. Look for any stray copper strands touching another terminal or the metal wall plate. Ensure each wire is straight and fully inserted.
HEADS UP: Using stranded wire instead of solid-core thermostat wire can sometimes cause connection issues that lead to relay chatter. If your installer used stranded wire, the ends must be perfectly clean and secured.