Muffler shops usually do the 'quickie test':
Burn a quarter inch hole a couple inches in front of the cat.
If the sound/tone/performance of the engine remains about the same, then the cat is NOT plugged.
Then, they weld it back shut.
Midas has done this for me several times at no cost.
You could do the same with a drill, then seal it again with a large galvanized (NOT stainless unless your tube is stainless) steel self-tapping/self-threading sheet metal screw. Make the hole in the side, not the bottom (so moisture that condenses in the tube bottom doesn't promote rust).
Some cats are honeycomb, and the rattle sound is a signal that the honeycomb has broken (doesn't happen often).
Other cats are pellets, so the rattle sound means nothing.
Bottom line: Does it flow or not.
Good luck.
-Rick