There could be several reasons why this person makes a groaning/moaning sound when they stretch. The person could have a chronic pain condition or making this sound could have become a habit. They might not know they are doing it. Or they might know but be unaware that it is disruptive to others. Or they might wish that someone would inquire about how they're doing. It might be helpful to first assume positive or neutral intent on his part and consider this behavior in the context of your overall relationship with this person. Do you otherwise get along well? Do they typically behave in a professional and respectful manner? You do mention being uncomfortable with it, so it may also be helpful to explore what it is that is making you uncomfortable vs. merely being annoyed like you might be by someone loudly slurping their coffee, crunching on an apple, etc.