Exhaust Smell in Cab
 

I own a 2006 explorer with a 3 valve 4.6. Despite multiple trips to the dealer, there is intermittent exhaust smell in the passenger department through the vents with windows closed. The dealer did replace the passenger side gasket on exhaust manifold. They also put ATF into the intake looking for exhaust leak from exhaust system with no results. They also tried to tighten up the weather striping where the hood meets just in front of the fresh intake vents.

When leaning over running engine, I don't smell it, but the smell still comes and goes into the cab through the vents intermittently. I appreciate any help or suggestions.