Is Humming Caused By Dementia?

My 84 year old mother began humming about a year ago. At first I asked her if she was humming a tune. She paused a moment and turned it into a tune and said, "It must be. Yes." She's not aware she's doing it. She also is hard of hearing, has some short-term memory loss and some dementia. I'm wondering why she's doing the humming. It doesn't bother me. But, I'm curious if the humming is because of her hearing loss or dementia. She lives with me and my husband.