oh, i feel sorry for you.
QX4 is a good automobile, you just got very unlucky,
i will also try to email them.
i don't blame you, but why didn't you try to return it the next day when you heard the sound?
also, i think you could've test driven it for a week before actually buying the car. Infiniti/Nissan lets you do that.
I wouldn't have done it either though,...... but next time i buy a car, i will remember to do this, so that i won't get into your position, thanks for sharing it with us.