Gracie does this, too. I have NO idea why. She doesn't bark when people come to the door. She actually doesn't bark at all. But if she thinks she hears something, she let's out a woof and a growl, and she will continue to growl for as long as she feels it appropriate (which is annoying at two in the morning). She doesnt even seem concerned about what she's growling at, though. She can he curled up in a ball under her blankets and react with a bark/growl. Dumb dog.
Abrams, Momma's Boy Chesapeake Bay Retriever and Cynder, Daddy's Girl muttly
~*~ DOB 4/11/13 ~*~ DOB 1/28/2006 ~*~