My dog is acting weird. He's growling at thin air. It usually lasts 5 seconds and ends in a grunt or bark.?

He's a 7 year old Shih Tzu who's never been a barker or growler. He seems non-aggressive when he does it. No teeth bared or anything. His behavior doesn't seem to be directed at anyone. He does it when I'm in the room and when he's alone. He does it when he's looking out the window or facing the middle of the room. He's been walked, fed, cuddled, and let out play today. I keep shushing him but he's persistent. What could be causing it?