Aww, I understand your worry! That "ghuk ghuk" sound during feeding could mean a few things, sometimes it's just baby adjusting to a new formula's thickness or the way it flows, tapi since he's not finishing feeds and the cirit birit just started, best to check with your doctor again to make sure the lactose-free is actually working for him or if something else might be going on. A few of us mums found that sometimes the gagging sound happens when baby's tongue-tie or suck reflex needs checking too, so worth asking your paediatrician about that as well!