Just changed spark plugs for low power and misfire from full stop to go. Also have been having this spark knock like rattle at 1800-1900 rpms. Now after plug change, got power back but I have a misfire at 70mph. Still have that rattle. I have a slow leak from valve covers and cats need replacing which I’m getting fixed soon. I have a 07 G35 HR. What could the rattle and misfire be from??

was there any oil in the spark plug chambers when you swapped them out? are you throwing codes? how do you know the cats need to be replaced? need way more info to help you diagnose this issue.

Did you buy your plugs from an Infiniti or Nissan bricks and mortar parts department? Because if you bought them from an online marketplace (EBay or Amazon), they could be fakes in very authentic-looking boxes. The fake ones work well for like a tank of gas, seriously, then you get the misfire.

Are you throwing any codes, maybe P0011 or P0021?

EDIT: And you have checked your oil level yes?

only throwing P0420 and P0430 sometimes. Goes away then comes back.
yes oil recently changed. Levels are fine besides the valve cover leak I have. It’s not a huge leak I’m assuming since my oil levels are normally fine.

yea actually there was oil on the plugs. The only codes I’m throwing are p0420 and p0430 occasionally. Goes away then comes back. And I can hear a rattle under my car that seems to be coming from the cats.

no the plugs I bought were NGK iridium from Autozone.

Good chance your cats are damaged, I would remove them and inspect with a flashlight/mirror or a LED borescope to see if there are melted spots or other visible damage.

im sure they are. But that’s not the rattle I’m referring too. Let me be a little clearer, so I can hear a rattle coming from under the car which I’m almost certain is my exhaust or my cats, but the other rattle I originally made this post about I can only hear when I’m driving with the windows up at around 2000 RPM. It almost sounds like something rattling inside of a can if that makes any sense. I was told it could be spark knock because the spark plugs are getting wet with oil but I’m worried it could be something worse. Before I changed the plugs and oil recently the noise was extremely loud and happening often. It’s not as loud now and not happening as often but it’s still there. Heres a link so you can hear what it sounds like. This video isn’t mine but this is the EXACT noise I’m referring to.