2004 Cadillac CTS SMELLS LIKE SOMETHING DIED

Hey everybody hoping someone out there has heard of this. Only when I turn on my heat/not a/c. After about two min it omits a foul oder, like fart bomb if u ever seen one a child, or a dead animal or rotton eggs. I brought it to the dealer and they looked in where the cabin air filter is, and found nothing, also had them look where the blower motor is, and nothing.

What do you think, im open to any ideas guys the smell is to much to dear.

thanx