I have about 48k miles on my 2008 350z and one day while heading to hang out with some buddies, I had did a quick 3rd to low 5th pull and let off only to hear a rattle become progressively louder on the highway which in my head sounded like I blew 5th... only that wasn't the case... When I had pulled over, the car had stalled out and I decided to start it up once more only the rattling sound was about 2x worse like there was metal bouncing around somewhere.

Fast foward after having it towed the next morning, I had some buddies tell me that I had "dropped a valve seat" which I heard can cause catostrophic failure within a cylinder and possibly spread through out the intake into other cylinders. I wanted to get a 3rd opinion on this diagnostic to see what the actual problem may be and what would be a rough estimate of repair costs? A series of unfortunate events prior to this troubleshoot have left me on a tight budget and unable to get to work so if it is recommended to DIY instead, please let me know. I can't find any thread directly related to this so my apologies if there is one. EDIT: Forgot to mention that there is NO CEL/Overheating/Abnormal oil pressure and the car just had a fresh Mobil 1 oil change.

 

Welcome and sorry to hear about your recent problem. It's only speculation to say that the cause of your rattle is a dropped valve seat, but it's also no exaggeration to say that this damage is not a DIY project, unless you are an experienced tech with a fully equipped shop to diagnose it. There's no way to accurately estimate what the cost of repairs will be, but you should put some money and time into getting an accurate diagnosis by an experienced Nissan tech to get some answers.