Horn goes off when trying to jump start?

[madmanjustice]

I have a car with a dead battery. I pulled the Tahoe up to try to jump start it but every time i connect the jumper cables the horn in the Tahoe goes off. This never happened with my 2013 Tahoe. Whats going on?

 

[Rocket Man]

Probably the alarm. Did you put the key in the ignition and cycle it on to turn the alarm off?