WARNING
Inhalation of exhaust gas is hazardous to your health. All exhaust gas contains carbon monoxide (CO), and inhaling it can cause unconsciousness and possible death.
Do not run the engine in confined areas (such as a garage) which are not properly ventilated.
If you think that exhaust gas fumes are entering the vehicle while driving, have the cause determined and corrected immediately. If you must drive under these conditions, drive only with at least one window fully open at all times.

Vehicles with a gasoline engine: the catalytic converter preheats for up to 30 seconds after a cold start. During this time, the engine sound may be different.