Why does the exhaust growl more aggressively for the first few minutes after starting

Originally Posted by RN SHRRK

I got up and at 'em early Saturday morning to go to Cars and Coffee, started my Z to let it warm up. Set off the alarm on my SRT. My next door neighbors came out yelling at me, saying they were going to call the cops for a noise complaint or some nonsense. They asked if I could make the Z quieter, and I said Sure, but I'm not going to. They said I should not be allowed to start my car before 8am. I only drive it like 2-3 times/month. I guess it's loud. Stupid headers.