Some women and men moan as a signal to let their partner know that the sensation feels good. Movies, television, and music present us with idealized sex scenes or lyrics of people moaning and panting at the height of passion. In reality, some people are vocal and may moan and groan, other folks may muffle any sounds with a pillow, and yet others may not make a single peep. Some express themselves by twitching or moving their bodies rhythmically as a response to sexual pleasure. Moaning doesn't only happen during sex.
Some people make small sounds while kissing, giving or receiving a massage, or snuggling.
Others don't have to be sexually excited to make a sound. You may, or may not, want to expand your moaning repertoire. However, if you're a screamer, you might want to be considerate of your neighbors and think about turning it down a notch. For more insight into moaning, read The Woman Who Loved To Make Vaginas Happy in The Vagina Monologues by Eve Ensler, or see the play, possibly playing at a theatre near you.