i haven't had the smell, but the source of some road noise on my truck could be the source of the smell you detect. my rear passenger door does not make a tight seal along the bottom. the weather stripping could do a lot better job than it does. i get a lot of road and wind noise from that opening. it could be that you might have a similar problem and under certain conditions, exhaust is drifting into your cab.

I also run my A/C a lot, which might keep my cab clear of any exhaust.