I raised five children that I breast fed, and never heard of this grunting syndrome. I even took care of children for many years before that, from my early teens (I am now in my late 60's) and have known hundreds of parents over the years without hearing about it or seeing it, so I don't really know if it's altogether normal, or is a condition that's increasing. It may not be dire, but when my daughter had her first baby, a nurse at the hospital chided her for not 'burping' the baby vigorously enough, and taught her to almost slap his back, and this child grunted like people are describing, until she stopped. It's very likely a tendency to experience nerve irritation of the esophagus or related areas. It could be natural for some babies, but for others caused by position, burping techniques, or formulas with too many foreign chemicals.
I read comments from mothers such as that breast fed babies tend to have colic (wrong, it's the other way around - formula and cow's milk fed babies almost always have colic), that a doctor gave one baby acid reflux medication (an insane idea - medication like that is not for infants,) and on this blog, one mother said things cleared up when she stopped milk and soy. Since a very low percent of the human population truly has lactose intolerance, I urge people not to accept popular nonsense about lactose intolerance, and instead understand that digestion problems with most foods are caused by a lack of getting proper nutrition, or by fatigue, or simply by not eating them often enough. Also, I urge people to obtain and read some old fashioned baby books such as an early edition of Dr. Spock, that will give you some solid, sane advice. It's hard to believe how much bad advice and drastic ignorance is out there these days regarding babies, medicine, and diet.