A few things to check:

1. Pull down the corner of the headliner at each hinge. Look up and find the hinge bolt. Check for tightness (this bolt tends to work loose). Your noise could be coming either from the hinge assembly or from a slumping tailgate touching the car frame.

2. Pop off the interior panel on the tailgate. Check the lock linkage for slop.

3. Check the alignment and snug fit (or not) of the latch. Easy way to check: wrap one or two turns of electrical tape around the strike. If that cures the rattle, then the problem is wear between the latch and the strike. You can replace the strike or even just the rubber mounts (which would change the mounting height of the strike and maybe take care of the noise.)

4. The tail gate has little stabilizing bumpers on each side at the frame. Have these gotten tired (and shrunk) with age?

On my 1980 I've done all these at different times to keep the tailgate (more or less) quiet.

Good luck.

Doug Harvey