She may be unaware that she does it so much. I had an assistant that would tunelessly whistle opera at times, years ago. He honestly wasn't aware he was doing it, so I (with his permission) poked or threw something at him whenever he started, he eventually stopped altogether!

Does she know she's doing it? If she needs natural light because of a brain tumor, might this also be something caused by that? Some tumors can cause tinnitus, (ringing in the ears) and some sufferers hum to drown it out.