Dear Gvol, There could be many possibilities a proper inspection would reveal the correct cause... But still mostly possibilities are give here..... This could be delayed ignition. It happens when the burners do not light off properly due to rust or dirt and the delay allows for a gas buildup. You need a service tech to clean and adjust properly. It could also be the duct-work expanding due to heating. If you stand near the furnace you should be able to tell if it's coming from the burner area or the duct-work.
Answers & Comments
Dear Gvol,
There could be many possibilities a proper inspection would reveal the correct cause...
But still mostly possibilities are give here.....
This could be delayed ignition.
It happens when the burners do not light off properly due to rust or dirt and the delay allows for a gas buildup.
You need a service tech to clean and adjust properly.
It could also be the duct-work expanding due to heating.
If you stand near the furnace you should be able to tell if it's coming from the burner area or the duct-work.