David:
You dont give enough of a description, but I would
speculate that it could be a loose hanger or connector
in the exhaust pipe or its shield(s) or something
related to the drive sections.
Suggest that you have someone with you, roll down the
windows, put your head out and listen to where it is
coming from. Then try moving parts in the area (dont
burn yourself) to see if anything is touching another
metal piece or the chassis.

Technically those guys at the Nissan dealership ought
to go WITH YOU and let the mechanic hear it for
himself. :-(





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