yah but i dont expect it to sound rotten...

I think the catalytic converter is the problem. Just need someone to answer this:

"Can the catalytic converter contribute to the exhaust note?"
"Why would you replace a perfectly good catalytic converter?"

if the cat is hollow, read punched out, it will sound tinny and a wet fart noise

yes, a hollow cat will make the sound different to a "proper" cat
just have a knock under the car of the cat, if it sounds hollow then its been punched through

drop in a 4g63 motor and have some fun

lol thanks creepin... if only...

yes wet fart, THATS what it sounds like. ok im going to get them to change the cat back (I have the original) and see if it makes a difference. they damaged the car the least they can do is humor me.