We have not checked for radon but it is bad in our area. We have a crawl space in the basement and I'm wondering if radon gas could come from the CS and possibly get into the furnace and cause a crack in the heat exchanger? Any ideas at all as to why a heat exchanger would crack in a 2 year old Lennox furnace? What causes odors to come from the furnace?

Yes you should check for radon if it's bad in your area, but I don't find sources claiming it can crack a heat exchanger.

The smell you are smelling may very well be due to the crack.

You need a professional out immediately. A cracked H.Ee is a serious thing.

Very odd, though, that it would crack after only 2 years. Sounds like a defect. I'm not familiar with modern furnace warrantees on the materials: Have you checked to confirm if yours might still be under warranty?