I had one when I bought my Z too. I did all the same stuff as you before I realized someone tried to jack up the car in the front middle under the radiator support. When this happens the bracket that runs from the bottom of the radiator to the top (bracket that holds the assy) gets misaligned. Mine was too far up so whenever the hood was closed the latch and the striker would squeak because there was no tension on them. I loosened up that bracket and moved it down (causing the latch assy to also move down) and the squeak stopped.
Good thing too because neighbors could hear the squeak coming a mile away and it really added to the hoopty value of the car