Furnace making clicking sound when it is already running

I change the tolerance level from one to two on the thermostat and everything works

If you have a Dometic single zone thermostat it needs to be initialized. This will stop the clicking.

System Initialization
A system initialization will need to be performed by the installer after the system is installed.
• Make sure the Single Zone LCD thermostat is in the Off condition. See page 4, “Quick Reference To Control Buttons”.
• Press the “+” button and, while holding it, also press and hold the On/Off Mode button for three seconds. LCD will show - - . Press the On/Off Mode button again to turn system off.
This completes the initialization.

Dometic ac and thermostats. On ours there is a setting for 1 or 2 degree temperature delay. Purpose is to keep furnace from cycleing on and off too soon. The relay on this is in the ac, and controlls the furnace also. If set to 1 second, you will hear the relay when it switches on and off before the furnace start delay let's the furnace actually comes on. Change it to 2 degree difference and the problem will co away.

my fridge does the same thing when on propane. It sits above the furnace and right under the A/C. Are you positive it wasn't the fridge?

Sicvilla. Did you have anyone look at it? It is annoying to me

When I first turned the furnace on for them it turned on and ran fine. It then shut off. When it turned back on the the heat was running fine but then they heard a continuing clicking noise. It would go click click click click and then pause then it would click click agian and agian will the heat was running.

I have also heard the exact same thing and it does come from near or at the AC unit in the ceiling. I have a 2014 FR3 and I concluded it is something new in the electronics. I have heard it when the furnace is running and it occurs just before the thermostat cuts off. It is not heat expansion and is a regular clicking just like you would hear from an igniter for a gas stove or grill. ON new models the AC unit blower and heating are tied together in some way.

Ok total guess: Was the flame cycling? Not exactly sure how to explain this... Also not sure about why this would be next to the AC. But anyway, my guess: maybe the furnace was cycling the flame off and on, perhaps they were hearing the igniter during the run. Was out for the first time this weekend myself and the first time I ran the furnace it ran for a long time to get everything warmed up. May this was what they were hearing. If so I'd say that it was normal and not to worry. But I worry about the next to the AC bit. That does not fit. Maybe one of the others will have a better idea. (Were you running the fridge on Propane? Did they perhaps hear that igniter? Or - what Korbe said?

Not sure about near the AC, but we will often hear the clicking sound of "things" expanding with the heat.