Johnny Cash humming in song "I Walk the Line"

I notice he hums for a few seconds before singing every time here - is it due to the chord changes? Is it musically significant? I’ve never noticed him, or any other singer, doing that.

Wiki:

I suspected as much. But damn, it still sounds good!

Johnny Cash could burp and fart his way through a song, and it’d sound cool.

That’s pretty much what he did during the worst of his addiction phase.

Thanks - should’ve checked there first.

Yeah, I love me some JC, but he wasn’t exactly perfect pitch.