I Have A 76 Year Old, Female Friends. We Often

I have a 76 year old, female friends. We often play bridge. I have noticed in recent months that she frequently hums--almost non-stop. Mutual friends have remarked on the same observation and we're trying to figure out why this old gal hums. Some people feel annoyed and distracted by her behavior. Do you have an explanation for such behavior. My "humming friend" seems to be unaware that she's humming. I'm assuming she has some kind of "tic" and can't prevent the humming, if she is even aware of her behavior. Please comment.