Popping isn't normal. There is a slight chance you're hearing the metal of the muffler/pipes expand as it gets warm, but the hangers and general isolation of the exhaust system from the passenger compartment should make that noise, if any, hard to hear.
What you were probably hearing was the sound of your engine failing from loss of oil. If the block cracked to let oil get into the cooling system, then cooling system fluid was also getting into your block. Hiss, pop, crackle, etc, when any of it worked into a cylinder then out the exhaust. Ouch.
Of course, I only think this in retrospect, 'cause we now know your engine was losing oil somehow.
OpalBlue.
