The Chihuahua has earned the title of being a yappy dog because they will bark and everything and anything. I have often joked that is why they have those big ears that stand up. The ears are little radar sonars. I say that lovingly. Elizabeth is probably the first Chihuahua that I have owned that didn't bark at everything. When she does bark it is something worth checking out.
I had one Chi girl who I swear had me convinced that my home was haunted. She would walk around growling not all of the time, but there were occasions. What spooked me was when it was just me and the dog and she would walk into a bedroom growling or the time she ran back the hallway barking and growling like she was chasing something. A friend then told me how dogs can see things we can't so I was about a phone call away from calling in the ghost busters.