Hey bro edwin, is that ur own exhaust smell or other car's exhaust smell (when u are closed to another vehicle) ?
If its later case, then probably its a/cond related lar, that the control valve didn't close back nicely (possible it happen after a lousy a/cond service), so its become semi-open, so will basically smell anything from outside ler, the higher the speed, the greather the airflow, and so is the smell.....