Yeah the cats are a known problem. A forum member recently sent theirs to Germany to have new insides fitted. Can't remember who though, but there is a thread about it. Not cheap, but less than brand new ones!
Are you familiar with the concept of a 'listening stick'? Get a large screwdriver and stick the handle end to your ear, and then poke the other end around the engine. It works like a stethoscope and amplifies the sounds in the area in which it is poked. Its very easy to hear valvetrain noise like that (ie the cam tensioners). You'll hear straight away if its big ends that way too - touch it on the block where the engine number is at the front.
Edit - I'm taking far too long typing replies tonight
