does it sound like it's coming from under the hood?
does it make the sound when you shut the car off?
is it constant?
does it do it when you punch the throttle?
Here are some possiblities:
Could be your auto leveling motor mounts (if you have an automatic)
Could be loose/slipping belts
you can go to
www.maxima.org and look in the classifieds for the 5th gen max... stuff always being sold, or you could post a Wanted To Buy thread (WTB)