Making a low, steady, singing sound with your mouth closed is called hum. You can hum along if you don't know the words to the song on the radio. Is this true? The earliest form of hum was hommen, meaning "Make a murmuring sound to cover embarrassment", which was first used in the 1500s.
Let's Get It Fixed!
HomeX virtual experts will assess your issue (for free!) and can resolve simple problems.