The cat will rattle like shit if gone bad if substrate detached.
You would soon know as you would hear all the time even with stereo up full whack.
It could be the dreaded coil breaking down problem, very common, or the very common misfire due to stupid rocker cover design, especially if early one.
Moisture collects after car cools down around plugs, NATURALLY, and causes tracking chrinically with usually bad pinking getting worse when hot.
Look at plugs for CORONA DISCHARGE, will be immediately obvious, can be cured permanently very cheaply(free) and easy.
tabetha