Ok I just checked my other 3 eggs and A & C are whistling back as well! I'm pretty sure I know the answer but I'm going to ask anyway, that means they have sounded/broken into the aircell doesn't it? Egg E's aircell end is still cool to the touch so I'm assuming it hasn't yet but I can't believe 2 of the regular shaped eggs are whistling back as well! I may have already asked but I didn't know eggs A & C were sounded so maybe its a lil different answer with normal eggs but how long is normal for them to be sounded before they pip the shell? I'm not rushing them just want to stay in the know lol, I didn't know it would be this nerve racking but omg it definitely is!!!
maybe .. maybe not.. i had one that peeped back at me 5 days before hatch and he hadn't broken through the air cell when he first started talking to me
when they first break through into the air cell part of the air cell will still remain cool.. it seems that as they get closer to hatch and turn around to prepare to pip that the air cell becomes a little warmer each day
for my eggs.. I had several sound 3 days before they hatched.. one of mine sounded then hatched the very next day.. so it all depends on how badly they want to get out of that shell and if they have absorbed the yolk sack yet.. if they have enough oxygen then they will absorb it and are ready to get out sooner than if they are oxygen deprived and breathing carbon dioxide