First thing, cats do need replacing at times, but if its just a ratle your hearing it may not be the cat at all,
One common issue is the mid mount breaking free of te exhaust pipe
Causing a loud and irritable rattle,
Check it by crawling under the car and pushing up on the exhaust pipe behind the cat (when cold to save your hand)
And by wiggling shaking pushing, you should be able to tell whether or not the midmojnt above the middle of the car is separated from the pipe,
If its not the push/wiggle on the car and if you can feel the catlyst moving inside the cat body then it needs a cat,
Oem or oe quality parts are the best, and deffinetly recommended, cheap weld in universals I would deffinetly avoid.