It seems to me that any expansion and contraction or friction related causes should have some sort of discernable pattern.
For example, wind from certain directions causing creaking or popping being especially noticeable as a roof surface heats up in the sun. Do you have any sense of what the pattern is? If you can determine the pattern it will go a long way in narrowing possibilities down.
If there is no pattern then it seems more likely to be something oddball and more ominous (like the fastener shear that Jay mentioned). I just can't imagine osb and foam making a sharp cracking sound under normal circumstances.
