As a kid, I wished I could whistle...just to carry a tune... still can't whistle.
My Dad & siblings are whistlers, so I'm used to it.
Now at work, depending if this person is on or off key, loud or soft can make a difference...if it bothers all...just tell 'em.
Once worked with someone who chewed and snapped his gum all the time with his mouth open. Way worse when he chewed bubblegum and would try to blow bubbles, they would snap on his face and then he would try to pick off the gum...now, that's a horror movie.