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

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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