I agree on taking it to the dealer. Many dealers sell Stillen parts so it would be quite hard for them to prove your exhaust caused this problem. Could be premature wear on the spark plugs but since they are ngk platinum from the factory I doubt it, maybe a bad O2 signal or something. Tons of possible problems.

As for the sound, I have always described it like you did. It sounds like steel marbles being poured onto a piece of tin.

ya GFL! any dealers near me would laugh at you, if you tried to get free work done bc of pinging with aftermarket parts that lean out your car.

Hope you have a better experience, but i sure wouldnt expect much.

depending where you are, try using some race gas to bump the octane. at least that would narrow down the possibilty of bad 93 octane gas.

LOL i would love to go to one of those dealers that would refuse to do work on my car when its under warranty. I don't think they would be satisfied if i called the nissan corporate and complain about the service manager. I laugh every time u guys say "the dealer refuse to fix a problem that is under warranty".

When you go to the dealer, you may also want to consider if you want to or don't want to tell them that you used fuel cleaners. I do believe the manual specifically says that additives/cleaners must not be used.