My mother "grunts" constantly.....is this normal?

Mom has this "grunting" habit that goes on most of the time. It's not loud or whiny, just a constant "uh, uh, uh" grunting. I feel it's just a habit that she's developed, because when I call attention to it, or she's in mixed company and is aware of it, she can control it somewhat. But now that her cognitive decline is worsening somewhat, she doesn't seem to be aware of it any more. It's more annoying than anything. Her doctor says it's just a bad habit when I asked about it, thinking it was due to pain, etc.