Exhaust Smell In Cab

[Bandit130]

Bought my truck in 2015. Had original exhaust manifolds and single exhaust. I could smell exhaust in cab when stopped at stop sign and red lights. Driving I do not smell it. Since then, both manifolds have been replaced. Most recent, Friday, had complete new dual exhaust from manifold to rear bumper and still have exhaust smell. I am at a loss as to where smell is coming from. Using AC or not, still have it. Any suggestions?