Why does my dog whimper when she eats a treat?

Sometimes my dog will whimper when she eats certain treats. I buy her normal milk bones and she loves those and eats them with no problem. Sometimes I will switch it up and buy different types of treats like the bacon flavored ones or the dog jerky treats, you know, the real fatty ones? I don't feed her those all the time because they aren't healthy, but she LOVES them. Only once a week usually. Every time I give her one though she whimpers while she's eating it... It kind of worries me and I don't know why she whimpers only while eating the different treats opposed to the normal milk bones. Any ideas?