Rattle in the Cat (89 5.0 GT)

After accelerating or coasting down there is the most god awful rattle coming from under my feet. I jacked the car up and pumped the gas (in neutral) and was able to isolate the noise in the front driver's side cat. I verified this by hitting the cat with a rubber mallet and it rattles inside (not the heat shield). The car has 70,000 gentle miles and the exhausts are near mint. I take it the inside of the cat has broken loose.
Questions:1. Is there anyway to get the bits out without having to cut out the cat? 2. Would you have to treat both sides to the same fix in order to ensure a balanced flow?
Thanks in advance.

Thanks for the notes.
I had the pipe replaced (with a used one) and sure enough the first Cat had come adrift. I was going to have a go at pounding out the damaged part and then take out the good one on the other side but the mechanic sawed off the blow tube so I let it go to the scrap heap.
We have pretty tough emission controls here and I have to get the car tested next month.
Again thanks:
Paddy.