My 2007 AT G35x makes this rattle noise from underneath the car when it's only in drive and brake is applied or reverse and brake applied. But nothing when in park or neutral. I can only hear it if I'm stopped at red light beside another car or stopped by a wall. There is no noise from the engine bay even when I left the car in drive and applied the safety brake and opened the hood. The noise is loudest when first starting the car when cold and gets quieter as it's driven and warmed up to the point where I have to crawl on my knees and have me head underneath the car to hear it. What could this be?

 

Check your catalytic converter heatshields. They're probably rusted and rattling right now.

 

Could it be anything else? Cause I've already ripped most of it off before but there is certain parts I've left.

 

You have to take all of it off. The top portion may be loose too.

 

Removing the cat covers piece by piece isn't the way to fix this issue! For a small fee any muffler shop will weld the converter shields for you. Doing it right will fix the problem and it won't happen again!
Gary

 

I got under there and let the car run when I heard the rattle I held a screw driver to various pparts of the catalytic converter and it stopped rattling when I held it this bracket or something that it's bolted to, towards the front of the car. Could it be a loose bolt or something?

 

Any ideas?

 

What did you find out? I have the same noise, but no rust on the heat shields. Maybe something inside like the actual material in the housing breaking up and rattling? Also i noticed when starting the car that my exhaust is fumes are stronger than usual.

 

I removed my heat shields and still had the rattle sound. I found out one of the cat cores is broken and is tumbling around inside.

 

Mine was the brace or whatever that is welded to the cats was broken at the weld point and was just re-welded.