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I haven't had that happen or heard of it... Did you tighten the hangers down enough? I used the stock clamp, but I don't think I had much overlapping pipe from the cat. I centered the exhaust, and I don't think I could have gone any more to the right, or there would be a hole/leak. (does that make sense?) I was worried, but all is well here. I may go back in a few weeks and make sure everything is still tight.

The exhaust probably does rattle/move some during use, so if it's very close to the diffuser, I guess it could vibrate against it? You should post a pict of how close it is... or maybe take the diffuser off and see if something else is causing the rattle?