Hvac unit pops after cycle of heater unit going off any info would be appreciated. Just replaced air filters

Sound like a flame pop or a metal thump? LPG furnaces are more prone to some back flash. However it could also be the ductwork "oil canning ". Try to narrow down exactly where you hear the sound and get back to us.

First time I had heard that term-- "oil canning ". I like it.
I tried to give you a greenie but my submit button doesn't work.

Does the unit make the sound when the burners shut off or after the fan shuts off? Is the sound a 'pop' like the sound of a cork out of a wine bottle or does it have a sound more like metal flexing?