This is Turtle.

It would be nice to know a model and serial number of it and about how
old is it.

Was it made before 1991 or made after 1991 ?

Does it have a vent fan that blows when the flames comes on. ?

now I will take it that it is a older model made before 1991 by GMC.

You have a fan/limit switch sticking. Change or read ajust the
fan/limit switch. if labor cost is high in your area , change the
fan/limit out with a new one.

Now if it is a GMC furnace made after 1991 or maybe 1993 that is
another horse of a different color. You need it looked at NOW and not
later. It could burn your house down doing this.

Reply with answer to the above or i answer e-mails too.

TURTLE