It could be an exhaust leak? with it running and cold take a spray bottle with dish soap and water, spray around looking for leaks then do it again when it is hot.
Look around for fuel leaks, and even coolant leaks, many people, because of the chemicals in our systems are susceptible to being irritated from things that we never were before.
You may need to just wash the engine and radiator to get the build up of crud, off it and that will help.
I worked as a mechanic and saw many people that were in the trade have to leave because they became allergic to oils, additives, or solvents.
Just a thought!