How old is the furnace? If it is 30 years old, it needs to be replaced anyway. The recent developments in Gas Furnace technology will make your new one so much more efficient and safe, that it should pay for itself in a few years.
If you are getting strong odors from your furnace, is it in every room, or just a few? Did you have the return ducts checked? Otherwise, you could have a cracked heat exchanger, which puts burned natural gas exhaust right into the house. ICK.
You should have a CO2 alarm in the house anyway, so this is a good way to check for a cracked exchanger, if the alarm goes nuts near a heat duct, that's probably your issue.
Otherwise, you could have water in your ducts that has rusted them, or a formerly happy rodent, or something else entirely.