It depends. Sometimes the only warning sound that a sounder of hogs give is a low grunt or two and they disappear. Other times they just scatter. Quite often the squealing that you hear is from pigs that are being shoved up to the front by the adults or it is from pigs being disciplined for some infraction of hog rules.

BTW: Wild boars have a distress call that is the equivalent of calling the cavalry. Every adult male hog that hears that call will come running. Saw it in action once from a tree stand when a big bobcat was hassling a sow and her 20 pound pigs. That sow let out a bloodcurdling scream. About one minute later a 250 pound boar came running out of the woods and disembowled the bobcat with one toss.