Ok, I got a video sample of Coco's random growling/barking at NOTHING.

Here she was just chewing on a bone and I was on my computer next to her. Suddenly she was growling and barking. I got my camera out right at the end of it. I do not think she was guarding her bone, anyone can take anything from her, and she will even bring things to me and ask to come up on the bed or couch to chew on stuff in my lap. Plus she is a picky eater, and doesn't care if our cat tries to steal things from her. It seems to me like she is scared of something, but I can assure you, while this episode happened, there were no strange noises or anything.

I am thinking I should show this video to my vet. Any other thoughts on this behavior?

Sorry about the loud noises I made with the camera..and also my dirty carpet. My cat sheds little white balls all over that and I have to vacuum it nearly every day!