I took my truck in for its first service at the dealer since they had offered the first oil change and urea fill at no charge. I noticed just what you are talking about on the drive home and decided to investigate once I made it in my garage. Low and behold, my oil fill cap was not tightened completely, pretty much just sitting there, and it was emitting a small stream of exhaust under the hood that I believe was getting sucked into the cab. Thanks dealer, I will handle my own oil changes and standard service from now on. (I have to anyways, egr delete). Not saying this is your problem, but worth checking. Good luck.