I have found some babies will *Click* or make a poping sound when the mouth
is very full of milk. I think what's happening is that the baby looses the
suction makes the *pop* sound and then gets it again.
Does this baby pop with every suckle or  in relation to the let-down?
If he pops with every suckle I would be concerned about a hidden cleft or
tongue tie.
Also does mom have sore nipples? Repeated snapping of the nipple would be a
cause. Mom may want to try Dancer hold to support the chin and lower jaw.
Hope this helps
Marie Davis