mxquattro wrote:My 09 G37Xs has a crazy rattle too but runs like a top. About 100K ago they diagnosed it as catalytic converter. I hear it mostly at idle, and bringing it up to 12-1300 rpm makes it go away so Im pretty convinced they diagnosed it right.
I just recently fixed this. I had to purchaser a stethoscope so I took the stethoscope to the Transfer case, Transmission & left catalytic converter and all sounded smooth and no rattle/knock....however, when I put it up to the right catalytic converter I could hear it clear as day. I finally got around to taking out the right catalytic converter 2 weeks ago and the honeycomb inside actually broke from its frame and was moving back and forth and in turn rattling inside the CAT. the sound resonated down there so it was difficult to pinpoint the issue exactly without the stethoscope. but once I used it to determine the exact location of rattle, the issue was the CAT. I purchased a pair from ebay off a 07 G35 and haven't had any issues since. I did the whole replacement myself and took about 2 hours from start to finish.