It sounds like "byou," in a really high-pitched tone. I can't even do it justice describing it in words.
He really likes to do it when playing video games. The sound has no purpose, but apparently it brings him some kind of satisfaction.
If he does it once, I just kind of cringe inside. If twice, I might make a face. More than that I usually put on a smile and ask him to stop. He probably doesn't even realize he does it most of the time.
I know others have the same situation. My mom can't stand my dad's snoring. My dad can't stand the sound of popping gum. Every couple has these little things they aggravate each other with.
I heard a story once about a woman that hated her husband's snoring. It would keep her up at night, and she would shove him to make him stop. However, after he passed away, she layed in bed, not able to sleep.
She realized that she could no longer sleep without his snoring.
It's amazing that something we find so aggravating can be something that truly is endearing us to the person we love.
So next time, instead of making that face, maybe we should just realize that someday we will miss that idiosyncracy when it's not around anymore. Take the time to tell the person you love them - which might distract them anyway - instead of telling them to stop aggravating you.