Bad smell from register

[Melbart4]

We live in a house built in the 70s. Tonight when the heat kicked on, there was a strong gas smell coming from the registers in the family room. Nowhere else. A couple months ago, we had the same issue when the heat came on, only the smell came from the basement near the furnace. We had someone come out immediately and he couldn't find anything wrong. Whats the *

[Freon]

If this gas smell only happens when the furnace is on I doubt it's the natural gas fuel you're smelling. You can use soapy water to check all connections on the gas line but you said someone already checked and found nothing leaking.

As a test, turn off the gas at the furnace then set the FAN control at the thermostat to ON from auto. The house air will circulate but no furnace burner will be on nor any gas to the furnace. If you still notice the smell, most likely it's a decaying small animal. Do you have mice droppings around the basement.