Baby Laughing Hysterically at Pacifier Noise

A baby laughs hysterically every time his mum puts a dummy in her mouth and takes it out.

Pacifier Pros
Some of the good things pacifiers can do for your baby -- and you -- include:

Lower risk of SIDS: Pacifier use during naps or nighttime can prevent sudden infant death syndrome. Doctors aren’t sure how it works, but if you give your baby a pacifier while she’s asleep, you might lower her risk of SIDS by more than half.

Satisfy the suck reflex: Babies have a natural need to suck. The bottle or breast usually meets this need, but the desire can linger even after the belly is full. A pacifier can help. Just be sure it doesn’t replace mealtime.

Encourage baby to self-soothe: Pacifiers can help babies learn to control their feelings, relax them, and make them feel secure. The comfort factor can be a double win: A calmer baby can mean calmer parents.

Source: https://www.webmd.com/parenting/baby/features/using-pacifiers