What does it mean when guy is humming a song when you both are alone?

Anonymous
My guy friend has been acting differently since last year. We're close friends. Only two of us go for lunch or cafe few times a month. Many times I heard him humming songs when we're alone. When people are sitting in nearby booths, he keeps his humming low enough so that only I can hear it.
Does it mean something?
What does it mean when guy is humming a song when you both are alone?
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