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Valve rattle or pre ignition knock please help

I have a 2003 nissan maxima with the 6 cylinder and during take off and going up hill i get this rattle between 1500 to 2500 rpm. No one can seem to give me a answer to fix it.

Usually you can tell by listening to it. They are distinctive sounds. I’m surprised an experienced mechanic can’t determine what’s making the sound.

What grade of gas are you using? Does the owner’s manual specify “premium required” or “premium recommended” for this motor?