Ive had the same with my DD2 and DS. They are big time grunters. With DS i have had him tested through a kenisiologist for allergies and he is intolerant to cocoa (BUMMER no chocolate) and eggs, so by avoiding this it is reduced. It can be just because they are learning about their bodies too. It is prob gas though so maybe look at what you eat, there are websites that will tell you what is gas causing when breastfeeding. If not breastfeeding it could be an ingredient in the formula.
I know with DD2 it did pass after a few months and just resolved itself. xo