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The problem is with the contacts in the fan relay, not with the thermostat. If it is an older furnace, the relay should be an individual component and is replaceable. If it is a newer furnace, the fan relay will be incorporated into the control board and cannot be replaced. Both of those answers above pertain if the furnace is a natural gas or LP furnace.
If it is an electric furnace, the first stage sequencer could be stuck closed because typically the first stage heat brings on the fan.
gas furnace??? check the start capacitor, put unit in ac mode and see if fan comes on(switch fan to on at thermostat) if it does not a fan problem or capacitor problem. when in cooling mode thermostat controls fan, when in heating mode, furnace controls fan
if you have a gas or oil furnace then the limit control is probably bad. If you have electric heat or a heat pump then the sequencer is probably bad. But first make sure the thermostat is not set to "FAN". The thermostat must be set to "AUTO". Many systems have a time delay on the fan to capture the remaining heat after it completes a heating cycle. Usually several minutes.
I suspect the sequencer is stuck. However, you may have a thermostat problem or a short in the 24V. If it is a heat pump then likely the reversing valve is not kicking in. There can be numerous causes for this and you should probably call a pro in for this one. Diagnosis is not such a simple thing.
You have a blower relay in the furnace that's probably bad. In the heat mode the blower is on a temperature relay, in cooling mode the blower has to come on immediately so there is another relay for cooling. The green wire on the thermostat feeds that relay, so if you go to the furnace and shut the power off and find the green wire from the thermostat that feeds that relay and replace it, being careful to mark all the wires before removing them, also the relay terminals have numbers for identification.
OK so its an all electric furnace no gas? I cant think of anything clicking on an electric furnace except the sequencers or the safety switches.
If this is whats your hearing then the fan should definitely be running.
Feel carefully around the fan motor (I assume this is in a mobile home). See if you detect any heat coming around the blower area close to the fan motor. If so the fan motor may be at fault or one sequencer may be out (the main one of course...) the thing is can you get the fan to come on by turning the auto switch on the thermostat (or if older model the Auto On switch on the furnace) to ON? If so you can "limp by and have heat til someone either checks it or you can go on deeper into it. but to go deeper you'll have to let me know you know where the elements are and that you have a volt meter. and feel comfortable using it especially on HOT wires.