I just had the engine swapped to a new VQ35DE imported from Japan. After doing so, this noise started happening. I sent the video to my mechanic who swapped the engine and he said it doesn’t sound like it’s coming from the engine. It definitely sounds likes it’s coming from under the passenger footwell and I can feel it vibrating in that spot if I put my hand where while the noise is happening. It only really happens when I rev past 3000 rpm and let my foot off the gas. I am going to check under the car soon. Btw the car drives great and sounds great apart from the rattle noise.

 

Check catalytic converter or heat shield