What's it mean when dementia patient nods their head and hums?

OMG I must have dementia, because I whistle, hum, nod and make faces when I hear a song I like playing in my head or on the radio.
If you think that is bad, you should see my hubby watching hockey.

In my experience as a RN almost all dementia/Alzheimer's patients have some type of behavior such as this. Many pick at things that aren't there, call out for their mothers/fathers, repeat same phrase over and over etc. My FIL does a sort of hum more like a beat to a song or something. I don't believe you will get a definite answer as to why people do this. It would be wonderful to know just what theyre thinking as they do these things wouldn't it? Maybe in their mind they are reliving some pleasant memory.