Brian I can tell you this. Last year my brand new truck a very tiny slight crack in the Exhaust Manifold no noise no nothing except for a very slight odor. Well at the same time my Pro-Heat went down one evening, was tired so cranked the engine up and went back to sleep. I do not know why but I awoke to a headache like I have never I mean never expierenced, if this was not carbon monixide poison I do not know what could explaine this.
This headache lasted until I got enough fresh air outside to get the carbon mx out of my blood stream. I had know idea that manifold was craacked until early this winter, all summer long cause it was warm it stayed swollen shut but when winter came the contraction from the cold made it show its crack. Got it repaired and glad its over. Ask others here, but I believe I would have died in my sleep had I of slept another half an hour.