Noise of Distress?

Hey, I have a question about bruxing...

My baby, Sailor, makes this sound every time I'm holding her...I'm concerned because as I understand it, it's a noise of stress or anxiety. Yet, she doesn't struggle or squeak or make any other complaint. She'll do it even when she's just hanging out on my shoulder. Obviously I don't want to be stressing her out, but I'm not doing anything, just chilling with her. Is there some other explanation for this sound, or something I can do to make her feel more secure?

Thanks for any info!
Here's a pic that she's being quite good for!