Hi. I have an 08 G35 Sedan. It has developed a knocking/rattling sound that seems to be emanating from the driver side of the engine in way of the exhaust manifold. Using a makeshift stethoscope it seems to be strongest around the driver side exhaust manifold. It is heard and felt in the cats pretty strongly as well and then tapers off as you travel down the exhaust. The passenger side has some too but not nearly as much. It seems more that it is just bleeding over to it from the left side. I don’t hear anything in the top of the engine with the stethoscope. It disappears or at least seems to above approximately 1500 Rpm. I also have a p0420 code now. The rattle is not there when I first start the engine cold. It shows up as the car warms and the idle drops from the initial high idle cold start. It then gradually gets louder and worse.

Below I have included two YouTube links: first is engine at idle. Second is revving engine

idle: revving:
Anyone have anything similar happen? I’m getting different diagnosis from Different mechanics (engine, manifold, cats are most common) and some are not able to be sure without tearing it apart. Hoping someone may have run across this and can point me in the right direction. Any input would be greatly appreciated

Regards

 

possibly the cats. Over time they do reduce in size. Mine are. i can hear mine at idle only and took off the Y pipe and can move the comb material back and forth. It's annoying but doesn't affect performance.