First remember that when you stop and the wind is blowing even slightly from behind the smell can enter the HVAC system. If it only does it once in a while that may be the cause. If not check the band clamps. If there has been a turbo removal the band clamps are not to be reused. Everyone does, but they are not supposed to be. Next is the rear EGR cooler gasket to manifold at the rear of the engine. Then the front EGR cooler to manifold gasket, then cross over tube to EGR valve. And of course don't overlook exhaust manifold to head leak. Look carefully for black soot marks using a good light and mirror as required. If anything has been apart and smell happens since then, always look there first.