Since this thread started I've been paying close attention to Zoee's barks.
If she hears something but isn't sure what it is she gives a low single bark.
If there is someone out front in our court (even if its the neighbors she sees EVERY day) she barks several quick barks.
If someone approaches the door she sounds like she is going to tear up whatever is out there, and her hair stands up on her back, and she growls when I tell her to stop. This is all out of excitement. She just can't control herself.