Cat Rattle?

Hi All,
 I just bought a 99 ex 2.3 5spd about a month ago and all way fine untill the other day. I was driving home and heard a rattle from under the car( it only happend when i was slowing down to stop or at idle). I tried looking around , checking things out, but couldnt find the culprit. Sounded like an exhaust rattle, but after wiggling this and that, no luck. So i took it to my local guy and up on the lift we found it was coming from the catalytic conv. When we bang on it it stops for a second. He said it needs to be replaced, anyone else ever had such a problem? How long do i have before i need to replace it? Im gonna change it, i just dont want to do more damage before i can order one.... Thanks in advance for any help.

one thing you might wanna do is double check to make 100% sure that the rattle is coming from the cat and not the resonator. i have a 94 ex 5spd, and something is loose in the resonator causing a very irratating rattle at a certian rpm range. (usually about 1500 to 2100)
 
if it is indeed the cat, i'm not quite sure how to help you. i dont have a whole lot of experience in that area.

Back when I had my S10 I had to replace my Cat on that and it sounds like you have the same problem.