my impression was that the noise was made where the panels were in contact with each other, or with the framework. Similar to how to metal on a barn roof pops as it heats up, the popping is where is rests along the framework and every now and then has to relieve the forces caused by expansion. I'll admit I don't have a great deal of experience with SIPs, but I could very well see where under-fastened panels will pop along the timbers as they heat and cool.

I was hoping you would read the sarcasm behind my earthquake reference. I agree with Roger, though, don't go feed a client a bunch of 'it could be...'s