Why do I moan during masturbation when I don't make a single sound back then?

So... I've started doing this activity since..well, long enough for me to forget when. Back then I would stay quiet and calm without A single reaction from my mouth. No sounds or moans or anything. But then recently I've started moaning and making sounds. I try real hard to keep quiet and calm like How I used to but failed. And recently it's becoming a problem like when I'm hanging out with my normal guy friends. Some little caressing on my arm can make me let out some weird sound that preventing it could make me feel weak and its creeping me out and scared that I may lose my friends. So.. Is this normal? Can anyone explain this sudden change?