We have a building with structural panels, similar to SIPs, on the roof. When the sun shines on them, they have been known to pop. It's even more pronounced now that there is metal on the roof. The popping is just the panels releasing the stress that builds up as they expand. Think if it like an earthquake, where the plates down gradually slip, rather they build up pressure until they let go all at once. The popping is like that, only less destructive.