My goodman gaspack has had two incidents of

Things to watch for when your heat exchanger is cracking....
1.Always kee the filter clean
2.Make sure the return air vents are clean and clear of obstructions
3.Your furnace could be Oversized for your house thereby running too hot.
4.The furnace can be "Overfired" basically burniing more gas and running hotter than it's supposed to be
5.It's always best to have your furnace taking fresh air for combustion from outside...basically you should have 2 pipes on your furnace ..one for Air Intake and the other for Exhaust
6.Sometimes some furnaces have a Bad Design and are prone to failure...Some Goodman have "Rings" basically large rivets that pop...this was a bad design however all the suggestions above should help but may not eliminate your problem