I have an 07 G35 coupe 6MT. Sometimes when I shift into 5th from 4th, i hear a grinding noise. This doesn't happen all the time, but many times already. Even when I shift slowly and smoothly, it would grind sometimes. What could this be? I'm still covered under warranty.

how many miles do you have on the clock; when was the last time you did a transmission fluid change? Could also possibly be a bad synchro.

I bought the G at 20,000 miles and now it's at 33,000 miles. Was it supposed to be changed at 30,000? Thanks

Your car is still under warranty, take it in as soon as possible and don't let them give you any crap about not needing a trans replacement.

GRINDS SOMETIMES! The dealership won't replace the trans if they can't make it grind all the time. Have Nissan (not Infiniti) service your trans, the cost will be considerably less and no warranty issues....keep all your paperwork!
Gary