All furnaces click at the beginning of the heat cycle, as this is the click of the furnace popping on. If clicking continues throughout the cycle or if you hear clicking but no heat is coming out, then you have an issue. The following can help you troubleshoot the problem.
1. Bearing Damage
Bearings inside the blower assembly will eventually wear out, a process that may be sped up from dirt and poor lubrication due to lack of maintenance. If the furnace is still blowing and producing heat, albeit with less efficiency, then bearings may very well be the issue. A heating tech can inspect the bearings to see if they can be repaired with cleaning and lubrication or if the bearing assembly will instead need replacement.
2. Dirty Blower Fan
Dirt and debris on the blower fan can reduce its efficiency, but you may only know it is there due to a rhythmic clicking noise while the fan is operating. The clicking is usually caused by a small obstruction being hit repeatedly by the fan as it rotates. Fortunately, a prompt furnace tune-up and cleaning should solve the problem.
3. Inducer Fan Balance
The fan on the inducer can also suffer balance issues, which will cause a clicking or thumping noise when the blower is operating. The fan may simply be out of balance and require adjustment, which is a quick and simple repair. In some cases, though, the fan may be out of balance due to damage. A cracked or chipped fan blade, for example, will throw off the balance completely. You must have the entire inducer fan replaced if this is the problem.
4. Thermocouple Issues
One clicking issue that also results in no heat at all is if the thermocouple sensor fails or if there is something wrong with the flame that causes the sensor to activate and shut down the furnace. The sensor works by detecting that an even flame is present to fully combust the furnace fuel. If something is wrong with the fuel valve so that no flame or a weak flame is present, then the furnace will be shut down before it starts, which results in a click. Sometimes the fuel valve is working properly but the thermocouple fails and shuts down the furnace for no reason.
Contact a heating service in your area if your furnace keeps clicking or fails to fire up correctly.