Re: Bad Fumes in Vent Air
It's pretty typical for exhaust studs to break off at the backs of the manifolds. They don't often get noisy though. Look for carbon tracks around the rear exhaust ports on the heads. On rare occasion, the EGR tubes can crack, but they generally do get noisy. It's worth looking for carbon tracks at the fitting off the manifold anyway. You'll at least be eliminating it as a variable.
One other thing to investigate would be to look for something burned to your exhaust pipes. We occasionally get a plastic bag of some kind that gets caught to the exhaust, melts, and smells really bad. After a while, your nose can feel kinda dry with that subtle burning/stinging feel as you breathe in really similar to an exhaust leak inside the car. If you find it to be your problem, some quality time with a razor blade scraper and a high speed wire wheel will get your odor problem solved.
We've also had an occasional valve cover leak that has allowed enough seepage to get out and burn on the manifolds. These ones have tended to get through the vents really easy regardless of hood seal.