Why does the horn blow when you open the door of your 2006 Chevy Cobalt?

The horn in your 2006 Chevy Cobalt may blow when you open the door due to the security system or keyless entry feature being activated. This is a warning signal indicating that the car is in alarm mode or that the key fob is not detected. It can also occur if there is a malfunction in the door switch or wiring. If the issue persists, it may be advisable to consult a mechanic or the vehicle's manual for troubleshooting.