Thump upon signal to turn off furnace

I have a Luxaire gas furnace. It is six years old in a newer build. I just installed a new wifi thermostat, a WIFI Sensi, formerly coming from a Honeywell. Since replacement I started to observe some different behavior. When the thermostat reaches its temp, I hear the "clicks", and then right after the furnace make a thump, deep enough to hear through the floor, and then one more thump almost right after. The fans continues to run for a minute, and turns off. This did not happen with old thermostat. The "thumps" also does not happen all the time. I try to recreate it by toggle the temp up and down but can not get it to repeat. Only when it has been on for some time, does this thump upon signal to turn off occur. And it does not happen all the time. By the way, this is for heat. What could this be? I know it is not the fan because the fan does not turn off right away. It is right when the thermostat clicks.

It never happened with old thermostat. I am about 99% sure I wired it correctly. All my wires were labeled, and I also wrote down what lettered slots they were going in on the former thermostat. With that, it was pretty much 1:1 with the new thermostat. The furnace works fine, has been for a few weeks, just this "thump"! upon signal to turn off.

Update: Listened more closely today. Thermostat reaches temp, first "click", then "thump". Then thermostat "click" again, another thump. Fans runs 30 seconds longer and it is off.

thanks in advance!!

 

Welcome to the forums.

Using two people..... one at the stat and one at the furnace...... try to identify the sound better. If I had to hazard a guess I'd say it's the burner shutting down that you are hearing and it shouldn't really be thermostat dependent.