Hi misecmisc,
I experienced the same a few times in my life. Weird, isnt it?

I cannot explain it, but i didnt care about it too much.
Observing your breath is a good thing to do. You could consider chosing a mantra, which is working great for many people. But thats a personal taste i guess.
Id suggest to not pay too much attention to the sound, as it is just a distraction from your focus. Just let it happen, like your thoughts happen.
Atb,
Phil