im dealing with this right now. i have had this rattle for about 6 months accompanied with power decreasing. I knew it was coming from the cat area so I took her off to take a peek

the wastegate flap is loose!! It sounds like a 12 speed bike coasting when the engine is DECELERATING ONLY!

my wastegate actuator rod definately needs adjusted so it sits flush to the turbo, but I have a question: How loose should the flap be in the turbo? mine has a fair amount of play which worries me because I have read and seen pictures where these come undone and fall into the cat

 

i have a 1999 aeb 1.8t with the same problem. my catalytic converter welds broke so i had to pull it off. i checked out the inside of the turbo and the swing valve that conects to the wastegate was shredded and looked like a saw blade. Its letting the exaust gas pass the turbo thus making my car lame. i m trying to find out if the valve can be replaced or what. the rebuild kits dont include this part.