Thanks for the advice guys. Keeping my fingers crossed that it's simple! I found the L6 235 troubleshooting pages in the FSM buried in the section on the V8 (not sure why), and there is actually a section for popping sounds. The possible causes listed include everything you guys have suggested, plus (gulp!) the head gasket. I'm REALLY hoping not to have to tear this down far enough to check that! I'll be taking Builder's advice and starting with the simpler stuff first.
