Popping noises

I am very concerned about the noises I heard. I noticed popping and
crackling when I opened the lid at around 100 F. I closed the lid and
waited a while. I eventually unloaded at 92 F. I can't imagine that
the noises are from cooling too fast except that the outside temp was
cooler than the kiln temp but other than that the kiln had been
cooling for 36 hours. It seems as though my temp readout and the
actual temp don't agree b/c the kiln fired hotter than it should have
(the 7 cone melted) but the readout was correct for ^6. I didn't have
a problem with the bisque fire so I wasn't expecting any problems with
the glaze fire----as far as temp goes anyways. I used the cones for
good measure, but I didn't check them like I should----LESSON LEARNED.
Anyways, does anyone know what the noises are?
LMac