Hello mate. Have you recently removed the precat? If not then it could be starting to break up but you'll have to remove the downpipe to find out. If you're going to do that then you may aswell remove the precat if it is still in.
Now.......if the precat is already out then it could be that the main cat is blocked by a bit of debris or just breaking down.
How old is the car and how many miles are on the clock?