About a week ago I started to smell a terrible smell coming from the heating vents on a forced air natural gas heating unit that I have. I read some articles about it maybe being a gas leak. I took my carbon monixide detector after making sure it was working all around the house and did get a response. I also read some articles about mice being in the vents and ducts. I took apart two of the ducts and looked inside and notheing was there. I then looked into the unit itself and where the heat and the exhaust from the buners is it smelled bad but no terrible. What is this smell and how can I fix it.

Definitely get that carbon monoxide smell fixed, however it should be noted, that the smell you are smelling CAN NOT be carbon monoxide. That gas carries no odor. It could be the smell of the gas itself. But that has a distict "sulfer" or slightly sweetened egg smell.

If you are smelling something like dead animal, then that's probobly what it is. It could also be sewer gasses creeping into your return. You checked the ducts... did you check in the air return. Those critters can even get inside of the heat exchanger in the furnace. To which I would say if he did, Im sure the little guy died a horrible hell of a death, and he will stink for awhile.