With the brake or e-brake on, an in neutral and at idle, rev the engine (blip or goose the engine) up high and remove your foot from the gas pedal quickly. If you hear the sound when your foot is off the gas and the rpm's are decreasing it's with a high degree of certainty that your cat has some loose cells or is loosing its adherence to the inside walls of the cat. You will hear it, it is noticeable. I was experiencing a rattle at start up and when coming to a stop, coasting or downshifting into gear, at certain rpm's. If I put the car into neutral instead of coasting or downshifting the gears, I did not hear it because the engine went to idle too quickly. Every time I rev'd the engine I could hear the rattle every time as the rpm's came down quickly. As Old Guy said, hitting the cat with a big rubber mallet is also a way of verifying the cells are loose in the cat. The method I described worked for me.
If it is your cat, I'd recommend 034's HFC and DIY. You can almost buy two of their cats for every one OEM cat. Your wallet will thank you and your face (big grin) will show it everytime you drive the car.