Hi, I have a 2006 g35 coupe that started exhibiting a rattle near the rear of the car when I cold start it and put it in reverse. The rattle seems to subside as the car warms up/reverse. The rattle also doesn't happen if I do a short drive, turn off the engine, and start it again. I did some searching and it seems like this could be something related to the catalytic converter/heat shields, but I haven't seen a video where the rattling was this bad! It'd be great if anyone can give me a second opinion, thanks!

Just in case anyone comes upon this thread in the future with the same sound - I got the issue diagnosed. Apparently there's some shielding near the exhaust in the rear (not heat shield, just some cover to protect from dust I believe) that had cracked and bent towards my right exhaust so that at a certain RPM the shielding/cover will vibrate against the muffler. The (cheap) fix was to just reposition the shielding slightly so it doesn't vibrate against the exhaust pipe.

Glad you figured it out, usually intermittent vibration sounds are something simple like that or cat shields.

Thanks for the information rowbear, my OE exhaust wasn't on long enough (2 months) to have developed that issue! That was very ugly sound for sure...Gary