My daughters used to make that same noise. She would then get really gassy and had a hard time burping and farting. I changed her bottle to Dr. Brown and it eliminated that problem all together. I always, and still think, that the baby is swallowing too much air when they're eating. I could be wrong though, I'm no doctor! ;-)
I've noticed when my daughter has that sound it's because she's swallowed air one way or the other (she's breastfed) because she let's a HUGE burp out..when I don't hear those noises she doesn't burp or very little..she's 3 months old (:
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