I have a heil furnace when the fan comes on it is very loud. I am wondering what might be wrong.

Which fan. Is it the draft inducer fan, or the main blower fan?

which fan. is it the draft inducer fan, or the main blower fan?

The main blower fan I think it controls the carbon monoxide

The draft inducer fan exhuast burnt gas out of the house through a vent(including carbon monoxide) the main blower circulates air within the house. So it sounds like your draft inducer is making the noise. Sometimes you can rebuild them(depending on if it's the motor or the fan assembly that is causing the noise),but you are usually best off to just replace the complete draft inducer, motor and all. Hope this helps. Let us know if you have further questions and we will do our best to help you out.