Ducane Furnace
Fuel: Natural Gas
The unit clicks on and the electric pilot heats up, gas starts flowing and firing into tubes inside it. This happens for a little less than a minute and then the fan kicks on. Almost immediately, the flames all puff out and I hear a minor thud as well as can see the cover push outward. Sometimes the fan turns off also, other times it runs for a short while before turning off.

It is freezing where I am, please help :(