Dog chattering teeth after sniffing baby’s feet?

We’re first time parents of a 4 week old little girl. Our dog Barney is a 5 year old half Staffordshire terrier (mother was a mix, looked almost like a mini golden retriever) we’ve had since he was 5 weeks old. We’ve been very cautious with interactions and have been making sure Barney still feels safe and loved even though our focus is obviously on the baby. He keeps a slight distance but will hang out on the couch next to us when we’re holding the baby. Sometimes he licks her feet and then looks like he’s chattering his teeth. What does this mean? I’ve read about dogs doing this to “savor” smells but not sure about doing it after licking.