The rocking and humming by itself is not concerning. It may be how she calms or soothes herself. It is more important to look at developmental functioning. Is she like other 5 year olds in terms of social, physical and cognitive abilities? Have teachers or day care providers expressed concern about her development or abilities to get along with others? If none of these things are of concern, relax. She will probably just stop the rocking on her own. As children progress through the elementary years they tend to conform and drop behaviors which make them appear younger or different.
It is good as a parent to be observant, if your child is having problems, early intervention has the best outcomes. So keep watching and questioning, but also just enjoy your unique daughter.
I thought the same thing but some reassurance is always good thank you for your prompt answer