See also:

Why didn’t my air bag go off in a collision? (Inflation and non-inflation conditions of the air bag)
There are many types of accidents in which the air bag would not be expected to provide additional protection. These include rear impacts, second or third collisions in multiple impact accidents, ...

Valve clearance
Inspect excessive valve noise and/or engine vibration and adjust if necessary. An authorized HYUNDAI dealer should perform the operation. ...

High-mounted rear stop light
In addition to the lower-mounted rear stoplights on either side of the car, the high mounted rear stoplight in the center of the rear window also lights when the brakes are applied. ...