There is always the most irrigating process, you can completely clean and reaseason the cooker, this would allow you to inspect it closely.
It would not make sense for it to be insulation, as that normally requires a lot of heat to ignite. If you use lump, it is possible a random piece of garbage snuck into your firebox
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"I'm Armenian...perhaps...but then again...maybe not..."