Once your milk has transitioned or "come in" (I dislike this term as colostrum is also milk so you milk has always been there) A baby who is getting a good mouthful of milk with each suck makes a small gulping noise with swallow. Some people describe it as a “GUH” sound.
When the milk is letting down/your milk ejection reflex releases your milk, there is a suck, swallow, breathe, suck, swallow, breathe pattern that results in a rhythmic gulp.
As Lactation consultants we like to see a ratio of approximately 2-3 sucks for every swallow for the majority of your baby's time at the breast. This combined with looking at baby's nappy output is a nice indicator that your baby is getting plenty of milk.
Check out my Instagram stories for a good example of what swallowing looks and sounds like.